How do you calculate the number of necessary games for a round robin tournament?

Round robin play involves each entry playing all other entries. For example if you have 4 teams entered, each team plays all the other teams or 3 games each for a total of 12 games played.

If you have larger number of teams, split them up into pools. Ie. 12 teams 3 pools of 4 each team playing 3 games. You then can take the top 2 best records from each pool and put them into a championship pool so you would have 6 teams playing 5 games each and the eventual winner would be team with best record.