Mailbox quotas rely on the package limitations of the shared plan since mailbox quotas get set by the user within the cPanel account.
For example: The Basic Shared Hosting package has 5GB.
The user under this package would be able to set mail quotas for their individual mailboxes up to 5GB.
If the mail quota is set to unlimited, then it would be relying on the overall package quota which is 5GB.
Of course, all of our shared/reseller packages have inodes limits: https://www.knownhost.com/wiki/shared-reseller-hosting#shared_packages
So 5gb or 100k inodes, whichever gets reached first.
Hope this helps.