We currently have two connectors installed in different on-premises datacenters. We are planning to add two more, one for each datacenter to properly load-balance connections.
In our current configuration, each server is handling its load sufficiently. I checked the statistics via perfmon using the AAD Proxy counters.
A recent issue we've run in to is a timeout problem with one of our applications. We published Atlassian JIRA which uses a connector in the same datacenter. All seems to work OK except when trying to display a service desk board/queue. It doesn't display any data. Opening the same page directly on the AAD connector server works as well as opening JIRA anywhere on our network. So we're convinced it's an issue with the Proxy and the JIRA application. We have the backend application timeout set to Long.
All other applications seem to work fine that we publish through the proxy except for a few functions through JIRA.