The boat loads top out at 14 or so. I've been on with 9 or 10 other divers a few times when the resort wasn't full. Typically over the course of the week some people (for example, me) will go their own way and the load will be smaller on a given day. Speaking of, do not skip the Mr. Bud, for my money one of the best boat dives, right up there with Calvin's crack and Mary's Place.
I counted the seats of the green boat (one their "standard" dive boats) a long time ago and I think there are 20. Easy Diver (cabin cruiser-looking) has fewer seats, I think, but I've only been aboard once.