Nah, like Panada just mentioned, it just gives 4 EVs depending on what Power Item the mon is holding.
Anyways, Panada also mentioned the limit for EVs. 252 is the limit for EVs per stat and 510 EVs in general for a mon. For Metang, you would ideally wanna go with 252 Attack / 4 SpD / 252 Spe. Also, be careful so Metang doesn't receive any unwanted EVs other than Attack, Special Defense, and Speed since you're fighting Charizard, Gardevoir, and Sceptile after it.
Also, if you need to see where you can fight wild Mons for EVs, which is quicker than fighting trainers imo, check out the Pokemon Reborn EV Training Guide V2 (Updated for Episode 15).