There are ICAP Maximum Connections recommended in the documents of ICAP deployment (link below), but however there are other environmental circumstenses that may have different optimal settings.
For the "ICAP Max number of connections", we usually recommend by clicking the "Sense Settings" to automatically configure the ICAP service using the ICAP server parameters. That will use the optimal number of max connection for that particular ICAP service.
You can also refer to below "Sense Setting" command in CLI:
To check if there are performance bottle-neck on the sensed setting, you can view Proxy AV appliance statistics for CPU, Memory Usage, ICAP Objects, ICAP Bytes and Concurrent Connections:
If you have more thank one(1) unit of ProxyAV box, you may refer to the below document for "ICAP load balancing explained":
There are also some recommendations in this document that you might want to take a look.
Please refer to page 76 on Default Threshold settings.