If you start a forum or a Q&A site. At the begining you would create the content yourself. At some point you get visitors from google.
What do you think: How many users du you need to run a forum or Q&A?
I mean that the visitors start to register and to contribute.
I hope to hear some real life examples or estimates.
Edit: Maybe the question is not fully correct. Probably it is also important to know other parameters like: "How many active topics are on the site" ...