SSH User Limiting

I have about 60 clients and all of them connect to SSH. Not always at the same time. I want limit like max. 30 clients can connect at the same time with their respective login ids.

How i can I achieve this?

Oh, I'm using Centos 5

[sajal]

see man sshd_config

found these 2 but dont think these solve your purpose

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.  Additional connections will be dropped until authen?
             tication succeeds or the LoginGraceTime expires for a connection.  The default is 10.

             Alternatively, random early drop can be enabled by specifying the three colon separated values “start:rate:full” (e.g. "10:30:60").  sshd(8)
             will refuse connection attempts with a probability of “rate/100” (30%) if there are currently “start” (10) unauthenticated connections.  The
             probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches “full” (60).

sounds like MaxSessions is what you are looking for...