I want to transfer my apple care+ plan from iPhone 11 Pro Max to 12 pro max is it possible?

I'm not sure you can actually transfer it. You may have to cancel the plan on your iPhone 11and the start one on your iPhone 12. But perhaps this article will help:

Transfer your AppleCare plan - Apple Support

AppleCare does not transfer between devices. It transfers from owner to owner of a specific device.

Purchase AppleCare for your new iPhone and cancel it for the previous one.