Killing 252 Paras' at Mt. Moon will give you 252 Attack EV's which at level 100 will mean your attack would be 63 points higher than it would be if you had gained no attack evs. This is because for every 4 EV's you get in attack, it will be 1 point higher at level 100 (63x4=252). For instance lets use Machamp as an example. Assuming you have an Adamant nature and a 31 attack Iv, but don't give MAchamp any attack evs, it will have 331 attack. But lets say you raise another Machamp and this time you have it gain 252 attack evs, it will gain more 63 attack points at level 100 than it normally would have, in this case giving Machamp 394 attack. So basically, evs can make up to a 63 point stat difference in a stat.