One is surely sufficient to make music but many of us are hobbyists. For me half the fun is being able to discover and make different sounds as well as doing mods. I think it would be easier for a guitar player to get by with one axe.
With bass you have 4/5/6 strings, frets/fretless, solid/hollow, electric/acoustic, roundwounds/flatwounds, singles/humbuckers. Technically speaking there are 96 possible combinations of those choices. Oops, I forgot short/long scale so make that 192 possible combinations.
Having said that, I'm starting to think enough is enough for me. I have an HH Ray 4 as my primary and it will make just about any tone anyone could want. But, for variety I have inexpensive versions of some of the other major types: one fretless, one acoustic, one short scale, one 5, and two Squiers (a P and a J) that are my main tinkering platforms - they spend more time on the bench than connected to an amp. So in one sense I have seven but tend to think of it as one of each kind.
With bass you have 4/5/6 strings, frets/fretless, solid/hollow, electric/acoustic, roundwounds/flatwounds, singles/humbuckers. Technically speaking there are 96 possible combinations of those choices. Oops, I forgot short/long scale so make that 192 possible combinations.
Having said that, I'm starting to think enough is enough for me. I have an HH Ray 4 as my primary and it will make just about any tone anyone could want. But, for variety I have inexpensive versions of some of the other major types: one fretless, one acoustic, one short scale, one 5, and two Squiers (a P and a J) that are my main tinkering platforms - they spend more time on the bench than connected to an amp. So in one sense I have seven but tend to think of it as one of each kind.

