I would define "too many" as a point reached where I would need to store a nice bike outside in the elements (or in a living area that my family would object to) because I no longer have the capacity in my garage.

And of course, it's perfectly fair to put hooks on the garage wall/ceiling to make that number as large as possible. But we're firmly in agreement that our 2-car garage will always be able to accommodate 2 cars.