Specific aircraft spots won’t be unlimited. Just the vacancies in the bases for those spots. So, the whole class couldn’t choose the 320 if they wanted.

For example, this past class had 21 total A320 vacancies. Bases were:

EWR - Unlimited
IAH - Unlimited
SFO - Up to 21
LAX - Up to 8
ORD - Up to 9

In this case, there are only 21 total 320 spots to fill. If you’re #22 in class and the first 21 people pick the 320, you’re out of luck if that’s the plane you wanted. However, of those 21 spots there are no restrictions as to how many can end up in EWR or IAH. If all 21 people who were awarded the 320 recently fell, hit their head, and accidentally chose EWR as their base of choice, then the remaining spots in the other bases will simply go unfilled.