I am currently expanding our number of Facebook pages in my Zendesk Support account but I receive the below message.
You have exceeded the max number of Facebook Pages allowed with your plan.
If you have the Social Messaging add-on or if you are a customer as of January 31, 2020 with an existing Facebook integration, you are entitled to link up to 15 Facebook pages to your account.
If you don't have the add-on and your account isn't grandfathered, then you can only add one Facebook page.
For more information, see the article: Setting up your Facebook channel