I think its pretty subjective. Personally, too many for me would be more than 3. But that's because we definitely couldn't afford more than 3, and I don't think I would be able to give the time and attention properly to more than 3 kids. We only have 2, and I'm not sure if we'll have another eventually, but if we do, it would definitely be it for us. No more than 3 for us.
Having said that though, I know several families that have 4 and 5 kids, and they seem to manage great with it. I'm always kind of in awe of how they manage finances, etc for that many kids, but honestly they are probably just way better budgeters than I am and have much less student loan debt, lol.