Select the map that you want to change th ticket count for and just below the selected maps box you will see two textboxes with the default values of 250.
You can modify these to what ever you want up to 1000 tickets per side.
You will have to do this with every map you are going to play...
Help Page of Server ver 1.1
Selected Map Ticket Count allows you to override the default ticket count for each side with your own values (up to 1000) for each side.