Teenagers don't commit to stuff like his because they're all desperately trying to decide if its cooler to go, or not to go. If there are people there, they'll want to go, if there arent, they wont want go. Your best bet is to full steam ahead as usual and make it clear to people that its still going to happen, regardless.

Worst case scenario, absolutely noone turns up which is very unlikely. But so what if it does, you're in an empty room and go "well that sucks", eat some cake and say to yourself " my friends suck, I need to find some better friends".

Next scenario, a few people turn up, you have a fun time, eat some cake, do what you do. In this scenario you know who of your friends are worth your time, and who arn't.

Third scenario, a few people turn up, you steam ahead as in the second scenario, people realise that its happening, realise they're missing out on a cool party and decide that they could make it after all.

The main point is to not worry about those other guys. Theyre probably more insecure than you are when you're confident enough to throw a cool party. Enjoy your party, you made it happen after all.