How do I remove devices from my iPhone 13 Pro Max?

If you mean something like an iPad or a Mac making and receiving calls through your iPhone, go to Settings -> Phone in your iPhone and turn off "Calls on Other Devices".

If that's not what you meant, please post back and clarify.

Regards.

Are you referring to this?

Make and receive phone calls on Mac or iPad - Apple Support

If yes, see Continuity features and requirements for Apple devices - Apple Support