I don't know if the questions is right, so how many online users shared hosting can handle? Can it handle for example 100 users browsing at the same time? What's the limits? If you ask it depends on the hardware, then I have following:
Dual Xeon 2.4 Ghz with 4 GB ram
I just need to know when it's the time to move. The other thing I cannot have a dedicated server( too expensive right now ) , is the a big difference between VPS servers and shared once?
There's more to it than just the hardware that the site is on. What kind of content does your website have... big db backend, lots of graphics, streaming audio/video or is it a lynx friendly all text website? What about the other websites on the server... do they have a big db backend, lots of graphics/audio/video, how many visitors do the other sites on the server get, etc.
There is no formula for it, there is no hard and fast rule, there really isn't even a generalized rule. If your site starts to become slow to load or the server becomes unstable, then it's time to move (or optimize your website). If it's fast and stable, then stick around.
There is no hard rule on how many users a server can handle at a time as there are too many variable involved.
As long as things run beautifully there's nothing to worry about IMHO. If the server does not respond well then you might ask to be moved to a less crowded server at the same host or you can "simply" change hosts.