My HubSpot+WhatsApp Integration account is not allowing me to add a new number.

You can only sync one WhatsApp number with one HubSpot+WhatsApp Integration account (1 account for 1 number).

Login or connect to your HubSpot+WhatsApp Integration account using the WhatsApp number synced with the account. If you want to integrate a new WhatsApp number with HubSpot, please purchase another subscription. If you already have a multiple number subscription, then you need to logout from your existing number and login using another number.