Im buying a Suzuki LTZ 250, which is going to need new tires in the near future. The stock tire size is (for the rear) is 20 x 10-9 would 20 x 11-9 fit on just fine? Which numbers have to stay the same?

 

The only difference between the two tires you are referencing is the width...the 20 is the height of the tire, the 10/11 is width, and the 9 is the wheel size. I would say to go to the wider size should be fine.
Mudslinger2

 

Mudslinger is exactly right its mainly the last number that has to be the same because this is your wheel size as long as you got room to go wider and taller the first 2 numbers dont matter much.