Short Answer 
To determine the number of App Service plans needed, understand that one plan provides the compute resources for multiple web apps that share compatibility in runtime stacks and operating systems. Given the listed web apps utilize different runtime stacks but can operate under the same OS, only one App Service plan is sufficient to host them all, adjustable based on performance needs.
Step 1: Understand App Service Plans
Before determining the number of App Service plans needed, it’s essential to know that an App Service plan provides the necessary compute resources for web apps. This means it dictates the environment where these apps will run, including the operating system and runtime stacks. Multiple web apps can utilize the same App Service plan if they share compatibility regarding the runtime stack and operating system.
Step 2: Evaluate Runtime Stacks of Web Apps
Next, list out the web apps alongside their respective runtime stacks to assess compatibility. Here are the web apps:
- WebApp1: .NET 6 runtime stack
- WebApp2: ASP.NET V4.8 runtime stack
- WebApp3: Java 17 runtime stack
- WebApp4: PHP 8.0 runtime stack
Despite having different technologies, all these apps can potentially operate on the same operating system (Windows or Linux) under a single App Service plan.
Step 3: Determine the Minimum Number of Plans
Since all the listed web apps can run on a compatible operating system, you can host them on a single App Service plan. This plan can be configured to manage various runtime stacks, enabling resource sharing. Therefore, the minimum number of App Service plans you need is just 1, which can be adjusted based on the performance requirements of the apps.