Q: For iCloud Keychain how do I change the phone number needed to verify the code being text to me? The number it was sent to is an old phone number of mine that I don't have access to.

Try this from another thread I seen.

On iOS go to Settings > iCloud > Keychain and tap to button to disable iCloud Keychain. (you will be asked to delete or keep passwords on device) i chose delete....

Then enable it again and in pop-up chose Reset iCloud Keychain, on confirmation pop-up chose Reset

In new pop-up  Use iPhone Passcode as iCloud Security Code? chose Use Passcode

Enter your passcode (4 digit) and then chose country and your new phone number

Although, to "Reset iCloud Keychain," I assumed meant to clear the data and essentially set up a new, blank Keychain.  Not so.  Strangely, after essentially bypassing the security regimen, setting a new passcode and verification number, I see my cards, passwords and other data have been restored.  Maybe it's all about the iCloud account and that master password.  Or maybe it's a bug.  With Apple, we'll never know.

Also have a look here: Get help using iCloud Keychain - Apple Support