I too actually struggle with this question often ... I'd be fine with almost any
one of my basses being my
"only" bass .... All but 3 of the basses in my collection are 4 string fretted basses as that's my personal preference.
Whenever I'm trying to decide which bass to bring to a rehearsal, recording session or gig, it always take awhile to figure out which one to bring. I'm not complaining about this, I'm grateful to have such a varied collection. I just think it would so much easier to grab my 1 bass and that's that.
I'm in my early 50's now but from 15 years old to about 30 years old .... I owned and played 1 bass ... a '75 Fender Jazz, it was all I ever needed and during those years I never even thought about buying or owning a 2nd bass, much less a whole collection. Truth be told, at that point in my life, I really couldn't afford to own more than 1 decent quality instrument.
I gigged that '75 J bass hundreds and hundreds of times over that 15 year period and that single J bass never failed me.
Flash forward 20 years later, I've got a good career, my kids are grown, I live by myself and can practice as often as I'd like and life is good. For the last few years, I've gotten to be in a position where I can buy whatever "reasonably priced" gear that intrigues me .... Some of the basses that I buy, I keep and some I resell if I don't bond with them. I currently have a collection of about 23 basses and I do love each of them for various reasons .... from an super inexpensive mod'd Squier / Warmoth PJ (it's ridiculous how awesome that cheap bass is) all the way up to a couple of beautiful Foderas and many in between including Rics, Warwicks, Sandbergs, Fenders, etc.
Rarely does a month go by that I don't think about selling them all off and only keeping 2 or 3 of the collection as I could easily be quite content with just 1 or 2 (or maybe 3

) ...BUT were I to decide to go through with selling them off ...
which ones to sell?! ... C
hoosing which ones to sell would be close to impossible. I've found that selling a bass that I own and enjoy tremendously is too difficult. Once I buy a bass and find that I really love it, the way it feels, plays, sounds, looks .... it's sooooo hard to get rid of it.
How many times have you read someone post right here on TB "
I should have never gotten rid of that"? - or - "
If I could, I would buy that "X" back in a heartbeat"
I often wonder if owning just 1 or 2 great basses and spending all of my playing time learning their subtleties, tonal varieties, sweet spots, etc would help advance my playing, even if just a tiny bit.
So to finally get around to answering your question .... I have no idea, sorry