This hasn't been moved to "other" yet, so I'll put my 2 cents in here.
It is going to be difficult to come to a consensus on exact numbers. When I was working full time in an orchestra, our contracts stipulated that we only got paid extra for "chamber music" at less than something like 8 or 9. I can't remember exactly now, but I remember we kicked up a big fuss over not being paid extra for the Gran Partita, but we didn't get anywhere. Managements will always want the number smaller, and performers will always want the number larger.
As far as IMSLP goes, I do believe that at 9 or 10, there is a label -- nonet or decet (sometimes modernized to "tentet"), so that terminology should be used.
At the moment, there is nothing larger than 8 performers in the chamber music page, and the gap between that and "symphonic", is covered by "other orchestrations (no less than 9) on the Instrumental (other) page. This is quite a gap.
I think the problem is that the "Instrumental (other)" page implies larger works because most of the things in there are symphony-based, or wind-section (i.e. Band), or string-section (e.g. a chamber orchestra). I don't think nonets and decets belong on this page, and I'm not sure about the elevenses and twelves either (don't know what to call those).
I also think the page with the symphonies needs to have a "chamber orchestra" category, for things like the Schreker Kammersymphonie that was recently added. At 23, this is very small, and on top of that, the strings are "one on a part" which is not normal for a symphonic piece.
While on this subject, can I ask what might be a silly question, & obvious to some, but I don't get it. Why do percussion duos go on this page rather than in chamber music? The top of the "Instrumental (other)" page stipulates "This genre also includes all purely percussion works regardless of the number of performers".
I realize that, historically, there was no percussion in the chamber music the Baroque, Classical, or Romantic periods, but 2 is still 2, and that's still small, fits in a "chamber" room or hall, and doesn't need a conductor. Looks like a duo, quacks like a duo, must be a duo . . . . . .