(I've done, and am continuing to do, estimation of that.)
There were 3.4 billion profiles, about 110-120 million ever posted anything publicly, but it was a much smaller core of 50-100k who were posting 50-100 times monthly (roughly 1-3x daily). And I figure a large share of that was span churn (the 50-100k was basically a month's worth of activity, and the attrition rate of active profiles was high).
We're collecting metrics of active G+ users through the shutdown, and that's, various:
4650 or so registered to the Google+ Mass Migration community on G+ (main migration community), since October 2018. (I'm a mod/owner).
12,000 at Pluspora (since Sept 2018)
38,000 signed the Change.org petition.
MeWe and a few other sites claim upwards of 10,000 G+ registrations, though they don't give sources or retention rates. Given that you have to sign in just to see content that's ... probably inflated.
Exodus communities in various places tend to be in the tens to 100s of members, from what I've seen (and again, I've been compiling sets).
This all skews heavily English-language speaking (though not necessarily native), and G+ was huge among Spanish, Arabic, Indonesian, Japanese, and Chinese speakers based on some indicators (mostly Community titles -- I've archive 8.1 million of those).
Upshot: if G Suite hits 10k or so members, that's an appreciable fraction of the core community G+ base, and might be more strongly motivated to use and like the product. Though of course, it's not public Internet.
(I'm frankly not a fan, but I do have a good sense of the numbers.)