so i m a little bit affraid : is there a limit for the number of users in community builder/joomla?
Nope.
have you experience with dynamic site on joomla/CB with more than 30 000 members?
The more you grow the better the servers you'll need to purchase. You should be moving to dedicated servers. You should trim out extensions with little optimization or optimize them yourselfs (CB it self is very optimized, so are our plugins). You'll at some point need to add new indexes to your database for most commonly requested queries to improve SQL performance (not something that can be done automatically, you should add these to database tables where needed when needed).
krileon
