There are proven answers to this question, believe it or not.
Well...sort of proven, anyway.
Okay, let's start with the most basic question: how common are magical beings, or humans who can use magic of some description?
If we're talking about small numbers--say one in a million--than there is almost no chance, so long as the being or person in question is careful, that they will be discovered. At that rate, the entire New York City Metro area, for instance, would have only twenty individuals who are in any way not normally human. Any oddities they cause, provided they don't do something physically impossible like shrink the Empire State Building, will just be simply disappear into the mass of information that is modern society. Vampires are a good example: occasional bouts of anemia that follow after a person met a pale stranger will likely not attract any kind of attention...provided the vampire can take steps to make sure that the person wouldn't remember being bitten, or that the bite marks wouldn't be spotted the next time they take a shower. If either of those is true, one in a million won't show up, but otherwise, you'd need active efforts to hide the truth, and there just wouldn't be enough of you.
One in a hundred thousand--which comes out to 202 in the NYC area--on the other hand, would require extensive cooperation, and very carefully planned coverups. Otherwise, you would start to have people get together to compare notes, and realize a certain degree of commonality to their experiences. "Hey, I just met the most fascinating guy last week, and now I suffer from anemia." "Really? Me to. What are the odds, huh?" That sort of thing. Provided the oddities are very small, and leave little to no hard evidence behind, you could still cover it up...but anything that is obvious, like a werewolf, would have surviving witnesses at some point, and that, alone, would lead people to start connecting dots that the Masquerade would not wish connected. And no, eliminating witnesses, or even erasing memories, would not help--both of those would leave statistical evidence behind, and, again, there would always be conspiracy theorists who would be able and willing to connect the dots.
If the number of beings involved in the Masquerade is as high as one in ten thousand, which gives you over 2,000 in the NYC area, it starts to become effectively impossible for any kind of Masquerade to be maintained. At this frequency rating, every single person in the country would likely know somebody who knew somebody who was supernaturally involved, resulting in a statistical near-certainty that you would start seeing concentrations of supernatural populations...and phenomena. Things like the Bermuda Triangle may seem cool to us here, but if you've got part of Harlem with the same reputation, that's going to make people start paying attention in a real hurry, and nobody likes to find out that they're not at the top of the food chain. At this point, even physical laws of the universe could only go so far when it came to keeping ordinary people in the dark, even if the differences are very minor and subtle--statistics plays no favorites, and it wouldn't be too long before the evidence began to pile up too quickly to ignore. Most people wouldn't know, probably, but the insurance adjusters, the police, and national intelligence agencies would.
At a rate of 1 in 1000--that's the same rate as the number of people with some mental disorders, by the way--any kind of Masquerade becomes basically a sick joke. Every single person we know would have had their lives touched in some way by the supernatural...and typically it would take place several times in a decade. At this point, you start running into the fact that chance plays no favorites, either--you WILL have people who have survived multiple, clearly unnatural events, that they know for a fact are flat-out impossible, and that they will not be able to rationalize. One or two such individuals could perhaps be ignored...but dozens start to become a problem. This would happen even with very subtle magics, by the way--a mechanic called in to fix a car engine that, for some reason, just will not start, but which suddenly starts as soon as he gets into it is going to note that fact. A real estate agent who keeps running into ghosts? She (or he) is going to realize there's something strange going on. And so on and so forth. Most won't believe, or will chalk it up to something normal...but enough noise, by enough people who can provide something that looks like proof, and the rest of us will start to go "oh, that explains X."
Moving on, the next question becomes: how easy is it to do something that violates the laws of physics?
If it is easy, or if it is something that can at least be done quickly, than forget it. If it takes me fifteen seconds to stop a car's engine--not a viable option in combat, I'll point out, but still something that could be done in a car chase--you WILL have criminals doing that in the middle of running from a bank robbery. That's just human nature. If it takes fifteen minutes, you'll occasionally see it show up, but not often. Fifteen hours, and it's unlikely to ever get used. I could go on, but you get the point. At a certain point, it just becomes easier and often safer to do something the mundane way, even if that runs a greater chance of discovery, which it might not.
Finally, you have the most obvious question of all: how is knowledge about this transmitted? How does a vampire learn vampire protocols, how does a necromancer learn how to raise the dead, how does a werewolf learn where to avoid so he or she doesn't get into a fight with other werewolves? All of that matters, all of it makes a difference, and makes it possible for a Masquerade to happen...but the more information you have, the more likely it is that your secrets will be revealed, and the more difficult it will be to repair those breaches.
