I have 2 phone numbers associated with 1 iCloud account, I want to remove 1 phone number and set that number up on it's iCloud account.

I had the same thing happen when I took a new cell plan with a different provider. So this is what I did went into setting then cellular than deleted the eSIM associated with the second number .

An Apple ID can have zero or more telephone numbers associated.

The telephone numbers aren’t all that relevant to your Apple ID, other than those used for two-factor authentication and account recovery.

Here is how to change the trusted numbers: Two-factor authentication for Apple ID - Apple Support