Right, I need to do a post on orc population numbers in order to estimate how many of them there were in particular clans now.
Firstly, we know that the Orcs are a violent and technologically primitive race, who were also oppressed in a hostile enviroment by various factors. Apparently there's already some sort of corruption or decline even before the Dark Portal, but there's also the Primals and Evergrowth, as well as the more powerful Ogres. Those are all external factors, but internally I might also remark that there's going to be some selective infanticide, as well as a generally bellicose culture. Tribal warfare is one of the most brutal types, so we're going to see a large amount of deaths from that too.
As such, I'm going to say that there was at most 1m Orcs on Draenor in recent years, based on population estimates for hunter-gatherers. This will increase and decrease at various points, and the Horde's formation is certainly a low point. Things evidently aren't going great, but Gul'dan poisons everyone with the Red Pox and accelerates the enviromental degredation by annoying the Elements. The Orcs hunt the Gronn to near extinction which presumably has some sort of metaphysical effect, then strip the land bare, even having to eat their warbeasts before coming to Azeroth. In addition of course the Orcs defeat the three other powers of Draenor, the Arakkoa, Ogres and the Draenei, which presumably further deflates their numbers. Let us imagine therefore that the Orcs have at least halved in population, down to 500,000, if not further.
During this period, Blackhand ordered various reforms, including accelerating the Horde's technological development, as well as ordering the clans to specialise. Strangely, the Frostwolves go from being one of the smallest tribes of around 50 individuals, to being a respected part of the triumvirate of Blackhand's generals. I shall assume therefore that part of that specialisation was for Blackhand to order that smaller tribes be absorbed into the larger ones. Blackhand deployed one part of the Horde first, the most disciplined clans and presumably a large part of the warriors, leaving the less disciplined clans (and presumably the non-combatants) behind.
Notable for being left behind were the Warsong, Shattered Hand, Thunderlord, Laughing Skull and Bonechewer. Comparably, those going through included the Blackrock, Blacktooth Grin, Bleeding Hollow, Burning Blade, Dragonmaw, Frostwolf and Twilight's Hammer. Absent from that list are the Shadowmoon and Stormreavers. It may be that these clans were small enough to either not be mentioned or simply be less relevant at that point. The Stormreavers are a clan almost solely composed of Gul'dan and his warlocks for example. There are also a large number of minor clans, either smaller or less prestigious, or extremely specialist like the Mok'Nathal.
I shall briefly note here that in older lore the Burning Blade are the most chaotic and bloodthirsty of all the orcs, and were left behind. Not according to newer lore which builds up them as samurai.
Again, to give us some numbers to work with, let's imagine that 200,000 Orcs follow Blackhand, and engage in his campaign, while the rest remain in Draenor, apparently degrading pretty quickly to Fel corruption and hunger. In the first phase of the First War, Blackhand moves steadily forward, taking high losses against the humans, and significant setbacks in the Siege of Stormwind itself. After the first siege fails, Orgrim assumes the mantle of Warchief, attacking Stormwind again, which no doubt is just as bloody as the first attack. At this point, let's assume 50,000 losses, leaving Doomhammer with 150,000, still a respectable force.
Doomhammer is at least moderately concerend though, and calls on the other clans in Draenor. They're busy fighting each other though, and only a small number come along, which explicitly doesn't include Grommash which is rather embarassing.
The Second War begins 4 years after the Opening of the Dark Portal. Doomhammer leads the Horde on a largely futile series of campaigns, including such brilliant tactics as smashing his face into Ironforge, and then smashing it into Silvermoon, and finally after he's walked off his headache, smashing it into Lordaeron. Unfortuantely this doesn't work. Doomhammer leads the Horde to traverse the entire continent, presumably proceeding at a fast pace and not spending much time tending to the wounded. Meanwhile at this point the Horde has no shaman, they're all either depowered or have turned to the Fel. Several clans basically wander off to do their own thing during this time, including the Dragonmaw, Bleeding Hollow, and Frostwolves. He ends up losing the war and taking significant casualties, losing half his force, which was already depleted in the First War when Blackhand had command. Doomhammer is defeated at Blackrock Spire, the rest of the Horde being imprisoned, approximately 6 years after the opening of the Dark Portal. In this time, I think we can conservatively imagine that Doomhammer has at most around 80,000 Orcs in his entire force.
Gorefiend and a few others, perhaps hundreds at most, presumably the Horde's remaining cavalry, manage to escape back through the Portal. Meanwhile, significant elements of other clans are able to escape captivity, notably some Blackrock and Blacktooth Grin in the Spire, the Warsong, Frostwolves, Bleeding Hollow and Dragonmaw. For the sake of argument, lets say we've got the best part of the 80,000 interred by the Alliance, and with tens of thousands escaped, either into the wild places of Azeroth, or in more collected forces like the Dragonmaw.
In what's quite an admirable move, the Alliance don't decide to just kill all the orcs. They intern them, where they sit around being sad for 20 years. Presumably mayn waste away, and there's very few children being born because everyone is very sad.
Meanwhile, the Orcs on Draenor aren't doing very well. They're weak enough that they don't have the strength to try again even if they wanted to. Instead, Grommash leads the remaining clans through the restarted Portal to go on an artefact hunt. Grommash sits around making a nusiance of himself while Gorefiend and Wolfbrother go after the loot.
Grom apparently just sits around until Ner'zuhl blows up Draenor, and then the Warsong and several other clans disperse.
I've remarked in previous infoposts about the Horde's modern clan structure, the experience of internment, and the broader implications of this series of demographic catastrophies. In general terms the Horde's population has suffered massive shocks, and events have prevented them from rebuilding till the present, with Grok being one of the first generation of Orcs born in the Horde. The point I want to make is that if they had high numbers, these have been devastated by warfare.
I'm not going to count the Clanless, as no sort of census will have been done on them, but of the larger clans and their consistient or cadet branches you'll have:
- Blackrock - Have always been the largest and most powerful clan, and retained enough numbers to split the clan under Rend and to start to specialise their forces again. Additionally, have enough forces to support cadet branches like the Demonsword Clan. Also allied with the Black Dragonflight, which provide them some techs which help. Relatively strong, and continuing to use the artificial aging techniques to get stronger, 80,000 individuals split between the Eastern Kingdoms and Kalimdor following Thrall and Rend respectively, and also including the random groups of people wandering about and hill clans.
- Frostwolf - 'True' Frostwolves relatively few, perhaps 2,000 at most, but having absorbed a large number of Clanless orcs under Thrall's leadership, one of the stronger political factions in the Horde, 30,000 individuals.
- Bleeding Hollow - Fought the trolls, but then stayed relatively safe and retreated to Draenor. Perhaps 10,000 individuals in total, split between a larger part on Draenor, and a smaller number of Azeroth.
- Dragonmaw - Stayed fairly independant, and somehow managed to keep a lot of their dragons. Active trade and alliance with the Blackrock, so population exchange and aging tech likely in return for air support. 20,000 individuals, all in the Dark Horde.
- Burning Blade - Mixed fortunes generally, with Neeru choosing to excise elements of the clan deliberately and specialise voluntarily. Likely small numbers, split between Orgrimmar, smaller settlements like Dreadmist, and then a secret sect in Desolace. Likely 8,000 individuals, with most of them in hiding.
- Warsong - Militaristic clan, stayed out of the First War, and free from captivity after the Second, likely causing them to absorb other clans more. Suffered setbacks like a significant number becoming Fel Orcs and dying on the Legion's side at Hyjal, but then were still trusted to be relatively independant by Thrall. Around 18,000 individuals, many in Ashenvale.
- Shattered Hand - Former slaves, likely absorbing some smaller clans by force of will, but also significantly degenerated in the modern day. Specialising as spies and assassins, likely <1000 individuals.
- Thunderlord - Vigorous, but decreased, no evidence of large numbers, specialised into beast tamers. <1000 individuals.
- Laughing Skull - Regarded as insane and treacherous even before drinking demon blood. Very few individuals in Azeroth, though some in Draenor. Likely fewer than 2000 individuals.
- Bonechewer - Another clan with a poor reputation, similar situation to Laughing Skull.
- Twilight's Hammer - Not a clan anymore, but a widespread cult. Not even specialisation in the Horde, almost entirely gone from open Orc society.
- Shadowmoon - Almost nonexistant, no independant settlements, will be gone in a generation. <100 individuals.
- Stormreavers - Destroyed, <100 individuals, with it being unlikely that anyone of them would declare themselves openly.
- Mok'Nathal - Nonexistant as a clan on Azeroth, <1000 individuals in Outland.
Consider these as informative, rather than fixed. Putting numbers to things is never a good idea in this sort of worldbuilding, but it does at least give me a starting point to specifically note down particular points.
Also, in reading about some of this stuff, apparently in current WoW orcs can now be priests and now actually have a lore reason to use the Light, also there's a really interesting and competently written quest series which goes into internal orc politics which is frankly just enraging. We had expansion after expansion of nonsense. Where was this writing during that? Extremely vexing.