Make this the last race of the season, stop filling slots when people drop out until you hit 45 teams, its only 3 teams that need to drop out, and stick to 30 per server for the entire season.
We've already seen 2 teams pull out IIRC, and considering we have a while till the season starts i'd be willing to bet that we'll lose another 4 of the 45 teams already entered and approved.
I also agree its not fair to refuse people the opportunity to race because of an oversight in planning, just stop replacing teams that quit until its at 45, you dont have to fill every slot just because you can.