Is it possible to create multiple web application on the same application pool? If it is possible what is the effect of this on the farm?
How many application pool can create?
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10 is the threshold for application pools but it is dependent on the resources on the server.
20 is the max supported amount of web applications.
Unless you really want separation of concerns, there's no need to go past a single App Pool with either multiple web apps or just use a single web app and utilise HNSCs.
Yes, it is generally best to have a minimal number of Application Pools, that is, a single App Pool for all Web Applications, and a single App Pool (mostly) for all Service Applications, WAS/PAS excluded in SharePoint 2013.
There is significant memory usage patterns that favor a single Application Pool approach, as well as some CPU performance improvement due to less JIT'ing that needs to take place.
I've documented this here if you want to read a bit more about it: The Expense of Application Pools.
HNSC, though, is entirely optional. In generally, I engineer farms within just two Web Applications: MySites and the 'general' Web Application. Some companies need more, but I always use a single Application Pool regardless for them.
There are certain limitations and best practices and highly recommended don't exceed those.
Every app pool and web application required heavy resources on the server, more app pools in farm more load on the server. A app pool size grow beyond multi gb size. If u have more app pool farm,s performance could be affected.
Yes, you can create multi web apps under single app pool. I would recommend to think about HNSC, if you just need a different URL for each site collection.