It's technically not a quota. It's a number of places that is set aside for future rounds, which are available to interstates as well as in-state applicants. This number is calculated each year very carefully and cautiously, based on current supply and demand and previous cohorts. When the second round offers come out, all the students who scored higher than the lowest ranking person from the first round offers will receive a place. Once all of these people who scored higher than the lowest ranking person from the first round offers get an offer, the rest of the candidates are then ranked in a single pool of applicants (not discriminating between first and second round interviews) and then offers are made down this merit list until there are no more spots left.
Essentially, the end result is that those who interview in late interview rounds have the same opportunity of getting a place as those in early interview rounds.