Irule to restrict number of concurrent sessions to 200

Here is the irule, you can modify as you need

 From http://devcentral.f5.com/wiki/iRules.table.ashx
 Limit each client IP address to 20 concurrent connections
when CLIENT_ACCEPTED {

    Set a subtable name with a standard prefix and the client IP
   set tbl "connlimit:[IP::client_addr]"

    Use a key of the client IP:port
   set key "[IP::client_addr][TCP::client_port]"

    Check if the subtable has over 20 entries
   if { [table keys -subtable $tbl -count] > 20 } {
      reject
   } else {

       Add the client IP:port to the client IP-specific subtable 
         with a max lifetime of 180 seconds
      table set -subtable $tbl $key "ignored" 180
   }
}

when CLIENT_CLOSED {
    When the client connection is closed, remove the table entry
   table delete -subtable $tbl $key
}

For the virtual server settings http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-implementations-11-2-1/30.html