You can only gain a total of 510 EVs spread throughout your stats, and only a max of 255 EVs can make it onto one of the six stats. Boost With Vitamins and Berries
You can often max out the EVs by level 20, which leaves the Pokémon free for use in-game; it will also be stronger than normal. Don't lose Pokérus. If your Pokémon get the Pokérus, always keep a few with the virus in the PC to pass on to future Pokémon you wish to train. Use vitamins early. 9EHQm.