I don't know how many are too many, but I know 10 are not close to the limit. At least I need a lot of bras, and I have small breasts so I imagine bustier girls are going to need even more bras ^^
I have an everyday seamless sports bra thingey, an everyday perfect fitting bra, an everyday perfect fitting push up bra, a thin no padding at all-nipples shine through bra, a no straps bra, a freebra, etc. Those I mentioned are 5 bras, and that is not counting real sport bras, doubles (I really need more than one perfect fitting everyday bra..) and lingerie bras. I don't count lingerie bras as everyday wear since they show through clothes when you have thin fabrics and/or tight clothes which I do a lot, so.
I own around 15, they're not enough as every time I search to find one to put on, there's something I don't like about it, so I buy others and throw the old ones and the cycle continues. It's ironic too cause I actually don't wear bras a lot (depends on where I'm going to) but still feel that I haven't got enough bras...
Too many? Next you'll be telling me there's such a thing as too many pairs of knickers. In my book there's no such thing but if they've wear and tear then I dump them. Same if I don't have matching undies anymore. I never don't match so one is pointless without the other.
BTW I don't lose the matching undies. That would be weird.