It's less about the total number of pitches and more about your vocal range and how well the pitches blend in with one another. So, you don't want them to be so close together that there's hardly any difference or so far apart that they don't smoothly transition.
My advice is to keep them between 3 and 6 semitones apart. 3 or 4 pitches with this spacing can easily cover a whole octave of your own range, which gives the voicebank a nice performance in that range while also allowing it to sound better in the octaves above and below it.
Of course, monopitch and dipitch banks can be high quality on their own, too; multipitch is just about preserving natural tone shift over one's range.