Any stat can only have a maximum of 255 EVs, and since EV boosts are only seen every 4 EV points, you can only train it up to 252 EVs (the next 3 EVs will be wasted).
Crobat is frail anyway, but what it does have is good speed and decent attack, so the EV spread recommended for it competitively would be this set:
Crobat @ Flying Gem
Ability: Inner Focus
EVs: 252 Atk, 252 Speed, 4 HP
Nature: Jolly/ Adamant (would be best)
- Cross Poison/ Taunt
I didn't recommend Brave Bird since that's an egg move and needs to be bred onto a Zubat.