If you're asking for the maximum amount of EVs a Pokemon can have, it is 252 in one stat (used to be 255 in earlier generations iirc, but the 3 extra EVs wouldn't give you a stat boost so it was removed), and a Pokemon can have 510 EVs total. This means you can max out two stats and have 6 extra EVs for another stat, although only 4 of those EVs will be useful.
Source: Pokemon Showdown and knowledge of Pokemon EV mechanics.
