What is the minimum number of people that should be involved since the beginning for a proposal to be successful, that is, become a full-fledged member of StackExchange?
The minimum number of people needed to build a site depends largely on how engaged your users are and how much participation they are willing to put into it. When you submit a proposal for a site, it is generally assumed you already have a community to build it. Area 51 cannot find that community for you.
Broadly speaking, you will need a community of at least 300-400 avid users (minimum!) ready to build your site. More is always better. Momentum is an important part of building up to a successful launch, so it's not realistic to start with fewer users hoping more will simply show up later — that is the primary cause of proposal failure.
The main hurdle is Definition where you'll need enough users sharing their most intriguing, real-world "example questions" to find 40 questions which will earn a score of 10 or more. Very basic, mundane, or poorly asked questions will have to contend with down-votes, and questions that don't fit our Q&A format can and do get closed. That's why more users are needed; communities who try to tactically robo-coordinate their voting to hit those minimum numbers with fewer users inevitably end up hurting their own efforts.
The second hurdle is Commitment where you'll need at least 200 users to "commit" to using the site if we create it. 100 of those users will have to earn at least 200 reputation on other Stack Exchange sites to move the proposal on to the Private Beta. If the proposal makes it to Private Beta, you will have about three weeks to attract at least 150 high-quality questions to become a fully listed public site.
The criteria to launch a site are not difficult if you have a community ready to move, but if you simply throw up a proposal hoping others will show up in time, it is highly unlikely that will result in a successful Stack Exchange launch.