There is a school of thought that you should make it slightly less than what you think you can sell out. Psychologically, if one always knows they can walk up on match day and get a ticket, it depresses advanced sales. If you think each match will sell out, you speed up the decision making process.
On the other hand I am a fan of more seats at lower cost creating the same revenue. Keep them low enough where the average punter or family can afford to go, and you'll always have a strong attendance. If you're not dross.