Sorry, there is no official document that sets a limit on the number of global administrators and domain administrators, it all depends on your requirements. But you could assigning the least permissive role means giving admins only the access they need to get the job done, to reduce the number of Global Admins. We could refer to the following document:
About admin roles in the Microsoft 365 admin center - Microsoft 365 admin | Microsoft Learn
Please note:
Security guidelines for assigning roles