I am continuously asked by management how many simultaneous users per 4-core ArcGIS Enterprise 10.5 license. Conversations with colleagues in the public and private sector put the number at around 100 - 125. What is the general consensus of those who manage an ArcGIS Enterprise system? When does performance really taper off? What causes performance to taper off?
A few more details include the following:
No advanced geoprocessing
Simple routes and Smart Editor
No in-house imagery
No additional 3rd party tools and widgets
Roughly 50 to 75 services
This is a question that will get you some very subjective answers. The load on your 10.5.x system will depend on a number of varying factors; such as number of services, load on the machine, supporting technologies.
You can be running a number of very complex processing services that will zap performance, or you can be running some home-grown cached basemaps that will just fly.
Normally you would use Capacity Planning Tool - GIS Wiki | The GIS Encyclopedia and depending on the number of users you will have and the amount of services determine the number of cores and hardware required. The link provides access to the process.
You can also contact Esri to do this for you. Professional Services normally does this kind of analysis for customers.
Sadly their methods don't always reflect the real-world solution. A lot of systems are interconnected and that makes that process only a starting point. I have done Systems Design Strategies a couple times (once from the ArcIMS/ArcSDE side, then more recently on the ArcGIS Server) and they only serve as a baseline, I have always padded the expectations to account for the things that CPT generalizes.