The amount of users the server can hold is entirely dependant on how active the users' sites are. You could easily host thousands of sites as long as they diddn't run anything CPU intensive (as in static pages). Dynamic pages, however, entirely different. Same goes with MySQL, as it can hold pretty much as many databases as you give it space to. I've never had any problems running multiple large active databases with MySQL.
As for how many simultaneous connections, again, that's dependant on how fast your uplink to the network is.
Apache php and mysql is depend on how well you have optimized it and totally affect on the performance of your site.
as well as it is also important to know that which other prime services are you using with this server like mail, dns and other..
"MaxClients, this is the number of Apache child processes that will be allowed, if your apache constantly causes the server to start using swap then lower this number, I recommend for most servers to set this at 8 divided by amount of memory in MB.
Apache Default 256
Recommend 8/Ram in MB (e.g... 8/1024 = 128Max Clients for 1GB ram)"
above is apache setting same way check for my.cnf setting and so....