Transferring to a new phone

I’m upgrading to an iphone XS to the 14 pro. I’ve basically never backed up my icloud and am worried it’ll affect something when I go to transfer data from the old phone to the new one. Once the data is complete, I hope to keep the old phone and give it away to someone else to use. However when I erase all data from it, wont it keep my number? How does that work? Will both phones forever have the same number? I want my new phone to have my old phone’s number but then what number will my old phone have?