with MWG7 a high number of connections are not as problematic anymore. If you still want to establish a limit you have the option to control the number of client connections:
Configuration -> Appliances -> Proxies (HTTP(S), FTP, ICAP and IM)
At the bottom you will find Advanced Settings and here you can specify the Maximal number of client connections that you want to allow on one box.
Is that what you are looking for?