When I initially read your question, I immediately answered to myself "more than 2, less than 15, per Medicare". But when I tried to find that in writing I was not able to. But I used that standard when I billed MH services for years.
What I did currently find is several insurance carriers define group size differently, some saying more than 2, less than 10, less than 12, less than 15, etc. It's a good question... and may be best answered by payer of the billed group service.