The other answers are wrong. **Yes you can get 255 EVs in one stat**. The thing is, the extra 3 above 252 are worthless as each 4 EVs makes one point on your stats.

There is also an overall maximum of 510. So if you have 2 stats and get 255 EVs in each you hit the 510 limit and get 63 points on each of those 2 stats (with 3 EVs wasted on each stat). But if you only get to 252 EVs on each stat, that's 504 total, meaning there are 6 EVs left where you can get 1 extra point somewhere.

Note that 2 EVs are always wasted, whatever EV spread you have.