Do Ev's stop exactly at 255? Like say i've been killing magikarps for 1 speed ev, if I kill 256 magikarps, I won't gain another ev?
Then it would look like 255/255/0?
Yes, you can't get any more than 255 EVs in one stat. This is to prevent Pokemon from becoming overpowered. Do remember that only 252 of the EVs are effective though, since 255 is not divisible by 4. So never train to 255, go to 252 and then that leaves 6 remaining EVs that you can put in another stat for one more stat point. This way, only 2 EVs go to waste instead of 6.