The most I know of one person convincingly voicing (without sample editing, that's a bit of a different matter) is eight? But she's a freaking wizard, and besides that generally pretends that two of those don't exist... So, we can safely say six.
Honestly, you can manage as many UTAU as you want to and feel capable of managing, but as far as voicing I really don't advice voicing more than a small few. Most people can manage to voice two, maybe three; four or five is possible for better voice actors, and if you're a wizard like the aforementioned, then do your thing. Or if you edit your samples I guess, but that still takes some skill...
HOWEVER. I don't recommend starting out with more than one or two banks--until you've made sure you know how to use UTAU and do a good job making a couple of banks, it's no good at all to start making a whole bunch. Then you end up with a bunch of UTAU that are all mediocre to crappy in quality and you can't even keep track of them all. :I
Personally I have a hard enough time keeping just THREE voicebanks updated, one of which is not my voice (and it's hard to get recordings for because we don't live together). But I never even have enough time to record for the two I voice so I mean;;;; don't be stupid, you can really only do so much.
I think one person manages like 20 UTAU that they don't voice, but I'm not even sure.
(I've also voiced three other one-off banks that I don't talk about; only one of those was ever released, but I consider it dead because I have no intention of working on it. Oh yeah, and I sort of manage a fourth bank that I don't voice that I just always forget exists.)
I'm wondering though, what do you mean "voice derivatives"? Like extensions/appends? Those definitely don't count as separate UTAU...