- always leave 16gb memory free for the OS
- if system is running above 80% cpu utilization then consider to add more CPUs
- if system is running above 80% memory utilization then consider to add more memory
- check the size of swap and paging file, and if swap file is using C:\ and file is large then move to a different disk to reduce disk contention.
- if C:\ disk is low on disk space then need to increase, always run Disk CleanUp tool to remove old Windows Updates files.
- Tune the ArcGIS Server Services min/max number of SOCs properly.
- implement ArcGIS Monitor , this is the tool to monitor your ArcGIS Enterprise deployments and ArcGIS Server Services, and it can provide a lot more information to proper tune the system.
I hope this helps.
Marcelo provided some good advice above but ArcGIS Monitor is a very expensive tool. You may want to start with the System Log Parser (SLP) which you can download for free from Github. Here is a blog post about SLP. You can get the latest release of SLP from Github. SLP will give you info on how your services are performing. It won't do real-time monitoring of your services but it will give you some insight into your server and services.
Bernie.