We have quite a few ArcGIS Server sites deployed... most of those are accessed through a central, standardized web-adaptor server hosted in IIS. As of version 10.3.0 only 11 web-adapotrs can be installed on a single host. The upgrade to v10.3.1 allowed 21 installations, but we are unable to install the 22nd with the default installation media. The issue is described at the following and there is a work-around that has worked for use for the past ~2 years: https://geonet.esri.com/thread/114296
Our largest web-adaptor server is sitting with 22 installs running v10.3.1 today, but is growing as our large organization centralizes deployments. I expect to be at 25-30 by years end. There really does not seem to be much of a performance impact having multiple web-adaptors on the same host; The only thing we've noticed is that the collctor workflow with many attachments chews up a lot of system memory on the IIS w3wp.exe process. We've stood up ArcGIS Server sites specifically for the collector workflow and set some limits on memory use on isolated application pools to handle this condition.
This really does benefit us as we can run web statistics from 1 set of logs and configure the 'front door' firewall settings on 1 host (for those deployments that have web-application firewalls with content inspection running).
Although the work around in place does work, I do have concerns about future support. Please provide the ability to install more web-adaptors on a single host. Maybe cap at 99 rather than 21?
Thanks for the consideration.