Looking at MUDStats.Com shows a "peak concurrent users" of 9000-ish for known muds (as known by mudstats). I assume they missed some; maybe the peak concurrent users is 14,000. For a popular pay-for-play MMORPG, you'd multiply this by around 5 to determine the number of paying players. For MMORPGs (and MUDs), multiply about 10x.
Nice. I wonder what other interesting MUD related sites are out there..