This question was just closed. While I understand why it could be interpreted as not constructive, I think that it is a legitimate question (though it should have been worded better). The question was essentially asking how many users does a site require to grow on it's own, which has an answer: it depends on the size of the site. I was in the process of writing this answer before the question got closed:
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.
Can that question be reopened?