Well, this site is currently supporting 36,199 users (look at the top left of the page - it tells you the number of members, and the number currently using it) And it seems to work pretty well...
It's not down to the technology you use to write the site, for increased users you need to design your website to accommodate them, and that means separating DB from user in both the software and the hardware. If you have a look here:
http://www.codeproject.com/info/about.aspx[
^] and scroll to the bottom, you will see what hardware it takes to keep this place running (and it is barely enough, some days...)