Hi Brian
As far as Symantec is concerned we view usage based on volume of mail rather than number of addresses.
One user could for example potentnially have 50 alias' but only send 10 mails a week and receive 20. Another user could have 1 address and send and receive thousands of mails per week but regardless of how many addresses there are between them that is still only 2 users.
The exact working on our usage limits are found on page 5 here: https://www.symantec.com/content/dam/symantec/docs/eulas/service-description/email-security-cloud-2017-11-service-description-en.pdf
Please note that this is how Symantec defines usage however I cannot commment on third parties such as resellers but I think that should help.
Regards
Ian Tiller
Tier 2 Senior Product Support Engineer