The way I understand it is that its as many as you can get. Just have to have life jackets for everyone obviously. I've had 14 on my 270 just to get to Hanky Pank!
There are different coast guard requirements on boat lengths. It's been a while since I had the class but I believe it's 29' where the requirements change. And my 38 does not have a plate but we do have the required one that covers polluting the waterways.