Ever since Gen 3, when the current system of EVs was put in place, the maximum has always been 255. The reason why some people say the cap is 252 is because that's the highest value that matters. Every 4 EVs gained, rounded down, will boost the stat by 1 point at Lv 100. Even though the actual cap is 255, those last three EVs will be ignored, like this:
252 EVs / 4 = 63 extra points at Lv 100.
255 EVs / 4 = 63.75, which the game will round down to 63 extra points at Lv 100, same as 252.