It depends if you have a small frame or a large frame.
For the small frame, you will only get 19 rounds with an Xtreme base pad.
As far as I know, there are now aftermarket springs and follower available for small frame to get more rounds.
For large frame you can get 22 rounds if you use the Xtreme base pad in combination with a spring+follower kit from Gregory Midgley.
http://ipsc4you.com/product/high-capacity-gm-magazine-kit/
But 22 is very stiff and it probably will not be reloadable. 21 rounds you can get for sure with this setup.
If you are using only the Xtreme base pad with original spring+follwer you will get 20 rounds.