252 Usable evs. (255 overall but 3wasted) Note that you can only have 510 overall stats And 2 evs are wasted You don't need to have 252 on one stat you can spread them out like hp100 atk100 def100 speed100 spatk110

The most EVS you can add to a stat is 252. You can do this to two stats to max them out. If you do 252 twice you should have 4 EVs left to invest. so in total you have 508 Evs to invest in a Pokemon if i did my math right