A lot of guys that make CBBs (cigar box basses) go with two strings because they think a neck without a truss rod won’t hold up to the tension of more strings. But with a calculator and a little common sense you can make a very playable 3 or 4 string bass without a truss rod by shortening the scale length and increasing the diameter of the strings. The 3 string 25.5" scale bass I made has a total of 45lbs of tension, whereas a 3 string cigar box guitar set up for slide playing usually has around 60 to 75 pounds of tension.
Personally I would never build (or want to play) a 2 string bass. You need at least 3 strings to be able to play a scale in one hand position and you’d have a hell of a time playing many iconic bass lines… and octaves for slapping… that goes right out the window. Even with 3 strings you’ve got to do a whole lot of moving around the neck.
Ok, that settles it, the minimum number of strings for the basses I build is going to be 4. Thanks @joergkutter. 