Because up to two stats can have max EVs, a Pokemon will usually have max EVs in the two stats it needs the most. If you have a hard time telling which two they are for your Pokemon, you can refer to this list. Find the term that best describes the Pokemon and use the corresponding EV spread.
Physical sweeper: 252 Atk/252 Spe
Special sweeper: 252 SpA/252 Spe
Physical tank: 252 HP/252 Atk
Special tank: 252 HP/252 SpA
Physically defensive: 252 HP/252 Def
Specially defensive: 252 HP/252 SpD
Fast support with no offensive capabilities: 252 HP/252 Spe
Your Mew happens to have a bad nature, as impish decreases special attack and Mew's best attacks are all special. If you really wanted to use it, then I would suggest that you use special attacks anyway, with the moves will-o-wisp, roost, psychic, and ice beam. Mew appreciates being able to move first, so give it an EV spread of 252 SpA/4 SpD/252 Spe.
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