Has there ever been a statement as to how many factions GW thinks is healthy?
Not as far as I know, but based on the number of releases they seem to be able to make each year the viable number could somewhat reasonably be somewhere between 24 and 30.
Edit: there was a thread about that topic on TGA a while ago, this was my post with the reasoning for the number 24:
I think 24 or so is a good number, mainly for product life cycle and production time reasons:
If they renew their rules with new editions every 3-4 years or so, they will need to put Battletome releases of most factions into that timeframe.
So if they release two BTs every three months, that is 8 books a year.
Thus all factions would have a new book every three years.
They will do other releases as well so they might not make it in three years for everyone. That's where rules updates in the GHB come into play (like for Seraphon, still playable with the Battletome from 2015. Well, mostly)
If the rules in the new edition don't drastically change they can maintain a faction playable with those small updates, so they might only have to release 6 Battletomes a year.
So yeah that is what I think. They make ~8-9 books a year for AoS, of which 6-7 can be Battletomes. If they want to renew their game every four years they can sustain ~24 major factions.
Edit: If I didn't count wrong we have 32 Battletomes since AoS release, making it 6.4 per year on average. Sounds like it could fit my above theory.