They put the extra students in the empty seats in the northeast corner and along the top of the east stands.
The only time I can remember seeing it happen was the Boise state game in 2015.
I think them saying there are limited spots and limits passes available is more of a marketing ploy to get people to buy earlier and to get there earlier than it is an actual concern of supply.
Just buy a roc pass. $150 is honestly very cheap.
They could maybe do single game roc passes to capture more market