Do I need to have macos 12.x on my machine to build macos app that can run on 12.x

You do not need to be running macOS 12 to develop an app that runs on macOS 12.

If you use something that Apple added in macOS 12, such as URLSession's data method, you won't be able to run the app on macOS 11. Avoid using anything added in macOS 12, and you can develop your Mac app on macOS 11.

While it might be technically possible, it might not be wise.
Could you be sure that the app would work correctly/reliably on 12.x, without actually ever running it on 12.x?

It would be a bit like developing an iOS app using only the Simulator, but not an actual device.
Possible(?), but not advisable.