20 seems fine, we only hav to do about this (though on our knuckles), how many do you actually have to do.

For a quick muscle boost you can do drop sets but these are quite taxing on your body and shouldnt be done every session. Basically do something like 2 x 10 reps (your 20 you do now) and then try another set of reps. Dont worry though how many you do, if you do 10 then great, wait about 15 secs then try for some more. The point is keep repeating until total failure i.e you can only do 6 out of the set. Then wait 15 secs and do it again. keep doing it until you cant even do 1. Then leave it 2 days before you do any more pressups and just do a normal lot of 2x 10 etc. Then next time do a drop set again. You really need the 2 days gap though for your muscles to repair or it really is pointless doing them. On the day you dont do pressups work another muscle group like your legs and alternate each day etc.