The number of phone numbers you can assign to a single Meta Business Manager ID is equal to the number of assigned WhatsApp Business Accounts (WABAs) multiplied by the number of phone numbers per WABA. This yields a maximum of 12,000 possible phone numbers (1,000 possible WABAs multiplied by 120 possible phone numbers).
Note that this example represents the maximum number of WABAs associated with a Meta Business Manager ID and the maximum number of phone numbers per WABA. You must make requests to increase both the number of WABAs associated with your Meta Business Manager ID and the number of phone numbers associated with each WABA. These requests are fulfilled if their corresponding business cases are approved.