I have a friend whose parents have something like 20 cats. I think only maybe 5 or so are indoor cats (plus two dogs). It's more of a situation where there was already a large ferrell cat population in the neighborhood, and she has been trapping them and getting them spayed/neutered. But then once she does that I think she feels obligated to continue to feed them. But at least they're getting spayed/neutered. And honestly, it would be sort of difficult to me to let them starve.
But to the OP- these people are eccentric, but not unstable. Very nice, generally. Actually, a lot MDC people would probably just find them to be normal, but the mainstream would find them a bit odd.
I was going to comment on how 15 sounds like way too many, but then I thought of my mil's best friend who does what this pp said. So I guess it really depends on the person & situation. No help, I know.