maximum simultaneous connections problem

hi list,

we have an installed gateway on a machine. radius and web services are
configured outside that machine.

our network is starting to grow. if there are only 400 simultaneous users
online, the server seems to work fine. But when the users reached 500 to
600, the server could not be able to handle new connections.

Do you have an idea what is the maximum simultaneous connections the
software can support? or do you have an idea on how to setup the server to
serve 1K to 2K simultaneous connections?

thanks in advance,
Rico