As of Gen Vi you can add a max of 252 ev's to a single stat.
Example:- 252 speed/252 attack and 4 sp.def = Total (510)
In previous gen's it was aloud to be 255 x 2 = 510. It's been downgraded and forced players to add 4 ev's elsewhere.
I think the above summaries the question, since it's quiet a short question.
Resource - exp.
Hope I helped!