i cant think of a reason to play more than 10 chars in pvp. you gonna onehit everyone anyway.
any char more than 10 would be overkill, or in other words, the overall performance of your battlegroup would drop ...

if you go for pve there is no reason at all to go for more than 25 accs.
if u think of modern raid design, i doubt you (or at least me) could handle more than 5 or a maximum of 10 chars and still beat the encounter...