I will try to reply to my own question, after some Google Earth hunting.
There seem to be three large transport units within PLAAF using Y8 and Y9. One of them had 17 planes as the max visible number of planes over time. Other two had 10-20 and 10-21 planes visible.
So an unit may have up to some 20 planes nominally?
There is also a separate detachment on another airbase, housing both the small Y5 and some Y8. It has usually 6 Y8. Now, it'd impossible for me to tell whether those 6 are actually a part of the other 3 big units and are thus double counted...
In four of the special mission plane units (so NOT transport plane units) there are way too many ordinary looking Y8 planes. And Scramble.nl says some regular transport variants are housed there. So all four may house several to perhaps as many as 10 of transport variant Y8/Y9. Again, hard to tell if they're ALL part of those units or are just a temporary detachment from one of the larger units.
Three more training bases have a few Y8/Y9 each.
And PLA aviation has a base with 4-5 Y8/Y9 of their own.
So, all in all, if no one has any better info, i'd venture to guess that there are anywhere between 80 and 120 transport variant Y8/Y9 in service with all the branches of Chinese forces. Frankly, that's WAY too broad of a figure set, but I don't know how to get info that's any more precise than that.