Yes, you do if you want those stats to be the highest they can be. EVs and IVs work seperately. A Pokemon can have a maximum of 31 IVs in each stat, which increases it by 31, and it can have a maximum of 252 EVs in each stat, which increases it by 63 (every 4 EVs increases it by 1.)
Also, a Pokemon can only have 510 EVs overall, spread out in whatever way you want over its 6 stats, without any stat having more than 252. But they can have as many IVs as you want overall, without any stat having more than 31.