C'mon, bass players, what's it all about? I mean for years you were content enough with four strings. Then in the 1980s some bright spark had a cunning ploy and added a low B string so as to be able to play bass parts with lows that otherwise only synth players would be able to reach (as was the fashion in the contemporary music of the time) and thus the five string bass was born.*
Then someone else thought "What if we add a high C string" and so the modern 6-string bass came into being.
And then after that it all went completely crackers. Someone else added a 7th string, and then an 8th, a 9th, etc, etc, and so it went on. It became a kind of rivalry between bass players - who could have the most strings.
Now, call me cynical if it makes you happy, but I'd say that 15 strings on a bass is just plain bonkers. And surely with that amount of strings, by definition, you'd have to move out of the bass range and into the treble, so is it really a "bass"?
[* OK, I've skipped over Fender's five and six string basses from the 1960s, but they weren't exactly big successes back then, although they are very cool instruments. I've also not mentioned instruments with the strings arranged in courses, such as those used by Cheap Trick's Tom Petersson: 12-string basses with 4 courses of 3 strings - 4 regular bass strings each grouped with a pair of octave strings.]