If I put 20s instead of 22s how many miles per will i lose

 

I of course can't give you a number, but the slight loss in top end will be offset by more torque on the low end (great for the trail or sand), and you will be faster off the line (my personal favorite thing to be!).

All you can do is ride it, and if you don't like what happended, then start trying gearing changes (bigger front sporcket or smaller rear sprocket).