Is there any way to see which seats were sold for previous games?
Check your arena page. That will show you the breakdown of seats sold in all your previous games
Also does it have anything to do with the level of my PR
This certainly helps but there are very different opinions on how much more revenue a higher PR manager will bring in in extra gate receits
Also what about the levels of Trainers and Doctors? Do they really matter?
They matter. Trainers train your players faster and Doctors help prevent injuries and helps players heal faster once injured.
I suggest you go through the manual and the FAQ guide on the help forums to familiarise yourself