You can effectively fill 510 points worth of EVs, at your choice. 4 EVs make 1 invested stat point, meaning any "EV leftovers" are usually 3 EVs or less that cannot make an extra stat point for your Pokemon.
252 EVs + 252 EVs = 504 EVs used
504 EVs + 4 more EVs = 508 EVs used
510 (Max EVs) - 508 (Investment) = 2 EVs left for use.
The leftover EVs are 2 in this example, since you need 4 to make a valid stat point.