How Many Application Pools and SharePoint Web Applications should we create?

I have came across many times , where seen people randomly creates new web applications using default settings as it easier to create, but have they think how this going to harm their Farm in near future?

Web application in SharePoint controls security, administrative settings and URLs for your SharePoint portal. You can create multiple web applications in a Farm where each web app has its own domain name, authentication mechanism and set of application services.  Web applications contained by an IIS Site with an application pool.  Once you have a web application, you can create one or more site collections and further sites inside it.

How many web applications do you need?  The answer should be three. 

When we review current state SharePoint implementations, we find that some customers have created multiple web applications to segment their SharePoint farm into zones.  However, this can create some significant performance impacts to your farm and to the underlying IIS server.

An intranet scenario mostly goes with Integrated Windows authentication. Partner collaboration goes with either FBA or Claims depending on the security requirements. For public facing sites, mostly it goes with Anonymous access + Windows Auth.

How many Application pool do you need?  The answer should be one. 

There is a nice article published that describes the impact of creating a new web application with its own application pool, a web application added to an existing application pool or a host named site collection. In testing each of these scenarios, they tested the amount of RAM consumed by adding these new resources:

• Adding a new web application with its own application pool: 400 MB of RAM consumed
• Add a new web application to an existing application pool: 59 MB of RAM
• Add a host named site collection to an existing application pool: 2 MB of RAM

Note that these numbers reflect just starting up an empty web application with no content, no users and no load.  Another article describes some more real world conditions of creating multiple web applications and/or application pools:

Click here to read this post

In addition, if you move to Office 365, keep in mind that there is only one web application in your subscription and you have limited administrative control over it.