ok how many users mybb can handle one time, i m expecting more than 2000+ users when i will start promoting this site
2000+? Wow, that's a big number for starting out, don'tcha think?
As for the capacity, it depends on how much your server can hold out. That's my own opinion, anyhow.
This isn't MyBB related as MyBB can handle infinite users at once. The number of users your forum can handle is up to the server you are on.