A six-string is the definitive electric bass, in my opinion.
All of the extra range is useful and adds value; you can play with guitarists in altered tunings without needing to retune yourself, you have easy access to an Eb, there's more scope to play inverted arpeggios or have your lowest note something other than the tonic.
With the high string you've got the ability to double lines in the guitar range, you can hit partial chords or arpeggios dead easy, there's lead playing or melodic lines if you do that. Plus the whole thing puts three octaves under your fingers without needing to shift hand position, which is probably the biggest advantage.
Anything more than six is an extra but isn't strictly necessary; the added range of a seven- or eight-string bass adds a lot more options and you can do a lot of cool stuff with it (I play an eight so I speak from my own experience) but it starts requiring different techniques and approaches at that point.
Six is a logical development of the electric bass, people have their fours and fives but I'm my own view and based on my own experience, a six does all the essentials, whereas a four only covers most of them.
Each six also comes with a built-in four-string, so there's really no downside.