To be completely fair that's 20 million (although it's bound to be much more by now) copies sold, not 20 million active players. 
Terraria Community Forums
Kazzymodus
StereOtaku
So lets say 1/4 are active players, I would expect 125,000 members.
MiltVala
Well most people don't really care for the forums themselves. I mean... the recent influx of members is primarily people complaining about console ._.