Serious answer: When having more basses stops adding to -- or even starts detracting from -- the joy and fulfillment you experience from playing. For pros, add considerations regarding basses needed to cover different sorts of gigs.
I had one bass forever. Then I got a backup. Then a fretless. Then different fretless basses and forays into five and six string basses -- all the stuff I was curious about but never tried in high school. I think I hit 10 basses. I don't regret the experimentation and the exploration -- in fact, I'm not done with it -- but 10 is too many for me to have long term.
Why?
I've always thought of myself as a player, not a collector. Starting to feel like a collector.
I feel stupid heading out to a jam and, on the threshold of leaving, agonizing over which instrument to take.
Something to be said for really getting to know a specific instrument.
Something to be said for putting all the energy that could go into scoping out and comparing gear into the craft of playing.
That said, I'll always have a fretted and a fretless. There's a good chance I'll still have a backup/alternate to my main four-string. And, though I've never found one I've truly bonded with, I am still curious about five and six-string basses. Not ready to stop exploring that, yet, but I would like to downsize. I'm down to eight now, hope to be down to five before too long and, from there, to refine what I have. I could see myself with three or four, and could live with two (if I put the 5/6 exploration on hold).
This is how I feel about my own situation. Do I think that some people have too many basses? Sure, but I don't worry about it too much -- or I won't here, anyway.
ADDENDUM: I acknowledge that different instruments have taught me different things about playing -- some of the funkiest lines I've ever come up with have been on $100 Squiers with 20 frets. That's not to say I prefer $100 Squiers with 20 frets for everything, but I did learn something playing the last one I had.