Each Marine has two progenoid glands. It takes five years for the first set to mature and then five more for the second to mature. So if start now we'll have three marines for five years, then we can bump it up to five in ten years, and in fifteen years we'll have nine. So every five years we can add the number of marines we added five years ago plus the number we added five years before that.
So, Space Marines are supposedly originally invented during the Dark Age of Technology and the process of creating them, maintaining the progenoid's genetic health, and creating a chapter is VERY poorly understood now. Technically, with Nid tech, would could hypothetically get there quicker if we had Biologis support, but this'd be something we'd want to invest time in for several reasons: the existence and purity of Chapters is highly controlled, so we'd want to be
very careful about our logistics and care in disguising them. Generally, their genetics are monitored once a year and ~5% of collected geneseed is tithed for purity testing and for use in future foundings.
As for what Foundings are? All chapters have official Foundings that are either prestigious or shameful due to how they turned out (for good and for ill). Marines' equipment and weapons also require a lot of logistical maintenance to keep going, so they're often tied to Forge Worlds in their sector. Now, technically they just show up and demand equipment, but social capital is very much a thing, so successful chapters will get more than failures. Obviously, Marines stand off to the side of other power structures, so raiding is something a chapter might do if it wants material and feels it can get away with it.
Why does all of this matter? If we start flashing around as having a new chapter, it's going to attract attention as to who we are and where we came from (and our gear), plus we'll need to provide proof every year that our Marines are pure. Now, technically there are ways around this, but we should be careful depending on where we start, because depending on the year, this could shine a spotlight on us if we're obvious about things.