Why don't you know the other groups? I don't even try to network effectively and I know most of them. I guess I am the senior pathologist, maybe that affiliations will come with time for you.
There are ways to find out who is who and these numbers. The government database on Medicare payments for one, the other would be the CAP directory. Start your net miles radius small and work out as far as you want. At the very least you can go to the hospital websites and do manual counts. I don't think anyone really cares and I am sure any medical directorship is paid based on at least 1 FTE even if 5 pathologists work there (peanuts). Things like the VA and locums would likely skew the numbers I would imagine.
I know the average groups in my areas are about 4 pathologists per group, some over multiple hospitals.