There is no magic number at which “how many” becomes “too many.” It’s more like a magic threshold one crosses at which point life goes from being “feline-friendly” to “feline-unhealthy.”
The author discusses various elements of healthy cat socialization, bonding, space and resources.
Source: PureWow, October 3, 2019. Link. Commenting on too many cats in one household, Justine Lee, DVM, notes it becomes really hard to monitor the overall health of each cat when you have to work overtime to keep track of who’s who. Urinary tract issues (and many more health concerns) are a lot harder to notice in one kitty when you have ten others stealing your focus.