How to recognize a server in a SharePoint server farm?

There are multiple factors that affect the capacity planning and server role allocation.

Performance is affected by the server hardware, the enduser hardware, the concurrency rate of site users, the throughput rate required, the response standards an organization deems acceptable etc. There is always tradeoff between performance Vs cost for acceptable throughput.

 

To summarize , as the number of users grows you add more WFE's and configure Network load balancer(NLB) whereas if performance of the services degrade add additional App servers.