You need to have reasonable number of members and thread/posts to go on for promoting, it all depends on the quality of your posts eventually.

I would say at least 2-3 new posts per day in each individual forum. (At least something new to read once a day). Start off with just a couple of forums and then subdivide them as they grow.

It really depends upon what type of site it is. If it's a support forum, it might not matter how many threads there are but rather how quickly a particular person's thread gets answered.

Yep, a community can start from 2-3 people, the most important part is your own involvement through thread participation and motivation to other members.