We have run into this question or concern a lot. The answer is it depends on what is happening in the environment and the business.
For each 20-25 concurrent users, we find that it is best to silo them onto a separate application server. This is typically by department so that should one department have an application error, it impacts them only – not the entire organization.
If there are 3rd party integrations in play such as FoxFire WMS, TrueCommerce EDI, KnowledgeSync CRM (CRM Studio), other integrations or SDK/API applications performing extensive automation capabilities then we strongly suggest they be on their own application server. This is so that should these components/solutions have a problem they are isolated from the application servers and client users … similarly, if the application server(s) being used by clients have any type of problem it doesn’t adversely impact the SDK/API integrations.