It's not so much 'how many members', but how many members who contribute good content that make a community.
Usually, the number of unique posters in a given month will be very low.
Meaning, it's the 2% user base that really drives things forward.
At least- this is based on a forum I started for a client after much research. Hmm..haven't checked in a while:
"3284 Registered users have made 1552 posts in 14 forums. There are currently 408 topics."
It's almost always maintained a 2:1 registered users to posts rates over the past 4 months.
# of Unique Members who posted last 30 days: 72
So, yeah- a tad over 2% keep the board going. Man, I guessed good.
Lurkers are the norm. I joined 10 Adult boards and may scan others once in a while, but only post here. Only so much time in the day.
In fact, this is how I'm chillin' out now- lurkin' about other boards- soakin' up info as much as my fried brain allows.