Tens of thousands minimum. Many adult websites get hundreds of thousands per day. It's very possible if you advertise it correctly. Of course, money is a factor.
Unique visitors. Afterall, you did ask "How many people", not how many page views... So there you are.
It depends what about you are talking about, if you are talking about a site with lots of free adult content - when yes, you need a really high traffic to say that it's big traffic. But if you are talking about simply website which purpose is to sell some paid membership when it could have less hundred of thousands i think.