I’ve seen a 7, 8, even a 12-string bass guitar, but are there any examples of bass guitars with even more strings?
I’ve only seen 4,5,6,7 and 12-string basses. The 12 is really 6-strings doubled. (The guy from Cheap Trick plays one, IIRC.) I’m not sure if it’s set up on octaves and unisons like a 12-string guitar, but you play it as you would a 6-string bass. The most range I’ve seen was a 7-string bass. The 8-string you’ve seen, is that doubled like a 12-string, or are all the strings separately played?
I can’t see any compelling reason to have more strings. I mean, hell, a 4-string Ricky is all you need. Even 5 strings I can understand, but at 6 it gets silly, and 7 is just plain overkill. Learn guitar at that point. 