Thanks everyone. I wanted to see what other people did, before giving my own answer.
For my Cawdor, I have 2 leaders, 5 heavies/specialists, 5 juves, and something like 13 gangers - so, 25 models in all. I consider them 'complete' although I wouldn't absolutely rule out making a handful more at some point for some additional options.
For my Van Saar, I have 14 currently painted, I just made 6 more and will probably do another batch of 6 or so later (having found that a nice size batch for painting). So, again in the 25-30 ballpark. (Given access to Techno skills, I'll probably need a few more specialists here.)
I realise that these numbers are larger than strictly necessary for Necromunda, but they allow me to have plenty of options and also potential to use the models in Xenos Rampant or even 40k, should I ever want to.