Party Favor question

The RSVP date for DS birthday party is 9 days before the party. I won't know how many kids are coming until then. Should I wait till then to order the favors? I'm ordering them from Amazon so it will take some time for shipping and for me to put them together. If I order it before, I will have to assume all the kids are coming so I have enough. The favors are sold at increments of 6. I invited 20 kids that means I have to purchase min 24. Between the favors and other personalized party items I'm looking at a difference of $10 per kid. If 2 kids can't make it to the party then I'm out $60. WWYD?

Party Favor question

I would try to order 3 sets... Im guessing that you will have a couple of no shows and maybe you can get a diferent favor for the other 2 kids if by chance EVERYONE comes. ... I would only get something different if there were two smaller kids or much older or some other way to distinguish, otherwise I think you have to just assume everyone is coming or choose a favor you can get quicker. Im guessing its too late to change the rsvp date to something earlier?