There was mention on another site that around 250,000 posts are made each week. I think it was a conference where Justine will be giving a presentation. My memory could be wrong, of course, about the number (not about the conference).
No info on how many individuals post, but there was some website which I found (now apparently not giving similar info/closed down) which showed some individuals made over 500 posts a week, while others may post once in a couple of years. It had a 'top 10' league of posters, but of course with some threads being word games, etc, those figures can be significantly skewed to show high numbers of posts without any qualitative analysis.
Even if MN has collected the type of stats you are curious about, they are unique to this site (as no other sites are likely to have 'shared' similar info) and commercially sensitive in terms of comparison with any other site, when attracting advertisers, and I bet there will be NDAs in place so MN could sue any advertiser who let slip details of MN's figures.
Some sites do an analysis but this is the only site I know which allows name changing (of course there may be others, but over 15 years I have used a few dozen sites and this is the first allowing changes, because one might change names to 'back up' an argument.
On here it has far more important use, of course, wrt sensitive issues, but no external monitoring 'spider' can know of there being changes made by an individual so would count each name as a unique poster and it would skew any figures. However, a spider looking at thousands of threads a week is going to be spotted on the MN logs and might automatically be blocked from access unless done in a very distributed way, costing lots of cash and of limited commercial benefit.
Most significant figures might be from the eABC doing an audit but even those figures would be commercially sensitive and not necessarily released by MNHQ if it even does use such an audit.