...
By default, meta only allow maximum 2 phone numbers to one WhatsApp business account, if you need more numbers, you need to reach out to meta support to increase the allowed number limits.
According to meta policies: “Business portfolios are initially limited to 2 registered business phone numbers, but this limit can be increased to up to 20.
If your business has been verified, or if you open 1,000 or more business-initiated conversations in a 30-day moving period using templates with a high quality rating, we will determine if your API usage warrants a phone number limit increase.”
...
Learn more about the phone number limits here.
C. Phone Set up Errors
This phone number already exists in your list of phone numbers.
...