SharePoint Online global storage limits

There is a limit for the amount licenses a tenant can purchase (which adds 10GB/license to the default 1TB) depending on the type of subscription being used (Or type of tenant that is).

For example:
Tenants that are using any of the Microsoft 365 Business (Basic, Standard, Premium) Teams Essentials, (MSA & Microsoft Entra ID) can only purchase up to 300 licenses. That would be 500k for Enterprise tenants.

See: https://learn.microsoft.com/en-us/office365/servicedescriptions/sharepoint-online-service-description/sharepoint-online-limits

If you have more than 500,000 users, contact a Microsoft representative.

There is no publicly stated upper limit for Enterprise tenants.

Some mega-scale organizations are believed to be in the 600k–1M+ user range, but those details are never made public.