You can see all that information here: http://southpark.wikia.com/wiki/Episodes#Summary
Season 2 was the longest season ever, with 18 episodes. Season 1 was the shortest ever, with 13 episodes. Seasons 3, 4 and 6 all had 17 episodes. Season 7 had 15 episodes. The rest all have 14 episodes and that is currently how every future season will be.