How many kids would you guess will actually come to my son's birthday party?

So my son is having his birthday party at Chuck-E-Cheese again this year and I'm inviting his whole class. It's just rude to not invite them all. BUT there are roughly 22 kids in his class! :-O Last year only 3 I think showed up. However, for some reason this year there haven't been any birthday party invites from his class in a good 6 months so I'm thinking maybe the parent are more opt to go since they haven't been to one in a while? What do you think? Makes me a little nervous because it's like $13 per kid. If I have to of coarse I'd just suck it up and pay for them all but OUCH. lol I know Chuck-E-Cheese is ridiculously expensive but he requested it and it's his favorite place and only goes for his birthday (or other birthday parties he's invited to).