How many characters are you taking?
How long is a piece of string?
It really depends on the army and then on the list itself.
My
LoV rarely have more than 2-3 characters.
My
SM maybe 3-5.
My
IG probably have 6+ in some lists and then less than 3 in others.
Edit: With that said, I'd say you have "too many" characters when you're reaching a point where your list would perform better with less characters and more units.
Example: Is it better to take 3 squads of X with character Y attached, or would it be better to simply bring 5 squds of X?