How many users do you need to reach the “tipping point”, when a site grows by itself? 

How ever many users it takes to make a high quality, active site.

If your site has good content, and is active, like Stack Overflow and Wikipedia, people will promote it, and want to join it and post content.

Of course, it takes a different amount of people to reach that level for every site. For sites like Wikipedia, is a general encyclopedia and needs tons of content to be popular, you will need a lot of people to generate that content. However, if you are running a small niche site, you will need less content, and as such less people, to reach that tipping point.