I assume you are talking about the express service from Schaumburg (or Yorktown, but I have never used that route). I think Pace runs as many buses as needed – I have never been denied a seat. Fare is $3 each way. The Schaumburg route is #282, the Lombard route is #779. Here is when the last bus departs:

Game Time / Schaumburg / Lombard
12:05 pm / 10:05 am / 10:00 am
12:20 pm / 10:15 am / 10:10 am
1:20 pm / 11:15 am / 11:30 am
2:55 pm / 1:05 pm / 12:45 pm
3:05 pm / 1:15 pm / 12:55 pm
6:05 pm / 3:30 pm / 3:30 pm
7:05 pm / 4:30 pm / 4:30 pm

For returning home – the last bus leaves Wrigley 30 minutes after the game ends.