when the season tool won't let you choose more than 25 opponent cars, you can add the missing cars manually in the .cch file wich the game creates when you start a season.
you can find this files in:
C:\Users\...\Documents\Automobilista\USERDATA\YourPlayerName
open with editor or word pad, scroll down to the line [CAREERSEASON] and fill in the opponents manually. it is some work, i know. be sure not to mix up the grid positions, each number is allowed only once in game, otherwise it will crash.
so:
1. create your season with the tool
2. start it, go to first free practise and quit without going to track.
3. leave game, open the right cch file
4. scroll down to the line [CAREERSEASON], search for your season there and fill in the missing opponents manually.
caution: if there is a track within your season, which does not offer enough pit boxes for your grid, you have to delete some drivers in the cch. - file for those races.