transfer phone number from iPhone 8 to iPhone12

A phone number is part of the SIM not the handset. All you need to do is move the SIM from the iPhone 8 to the iPhone 12. Both phones use the Nano SIM.

Hi - If you are in the USA, there may be a bit more to it.

See:

https://support.apple.com/en-us/HT210655

I see, that would be the process for iPhones 8 to 12 for SIM to eSIM?

Would moving the physical Nano SIM across still work?

The article also covers physical sim transfer, but links to further steps.

Damnit - no it doesn't, that is the wrong article :-/

You can move the physical SIM to the new phone and it will transfer your number and account. The eSIM will still be available for a 2nd line if you want one.

It is possible* that I have completely misread that article. Sorry.

*If "possible" can include 100% probability :-)

Don’t worry about it. That’s one reason there are lots of other users in the forum 😁