4 String or More?

The 4 string bass might be the most popular.  It's tuned like the low notes of the guitar.  In that case, a guitar player can at least tune a bass.

It wholly depends on what kind of music you want to play, and what tuning(s) the guitarist is using. Unless you’re using like a Fender Bass VI, which is basically a guitar tuned an octave down, I doubt you need a normal 6 string bass to play, say, pop-punk