They'd most likely wait and see how many posters they need, then get that printed and sent out to whoever. The 20 people limit would only apply to the $10 contribution. They'd have a poster for each $15 contribution on top of the 20 for the $10 contribution.
Seems daft to me - why would they limit the number of people who are allowed to advertise their game for them, for the sake of $1 or whatever each poster would cost. I mean, if your a bit late and contribute $10, you get nothing? - I hope this is just an example, and nobody is actually doing that!
I can understand if it was say, a figurine of one of the game characters, then limiting that to 20, first-come first-serve is understandable, but it would have to be a bonus thing, a limited edition poster of a game that needs kickstarter is not a good way to spend $10... spending $10 for the chance to own a poster is ludicrous.

I am the one who knocks...
