To answer my own question, it's 6 (5 if you don't count Noah Daniels). 1. President David Palmer 2. President John Keeler 3. President Charles Logan 4. President Wayne Palmer 5. President Noah Daniels (acting president) 6. President Allison Taylor
Whoever was President before Palmer I in season 1 (not sure a name was ever mentioned) Palmer I Dude before Logan (died in that plane crash) Logan Palmer II Taylor
There aren't really any plot twists... or at least none that I've seen. This season looks more compelling than the last two IMO. Can't wait for episodes 3 and 4 tomorrow.