How to give one time only camera access to Skype app I want that everytime a videocall starts on skype, MacOS asks for my permission to give access to Skype. I don't want to give permanent camera access to Skype. 

I am on macOS Catalina 10.15.5
You can just give it access for that one call and then revoke its access after.

To change the camera permissions on a specific app who has requested for it:

Settings > Security &amp; Privacy > Camera Tab and then click the check mark next to the app you want to change permissions to.

You may have to click the lock icon on the bottom left corner to change these settings.