If people really using 17GB of email, you can have 10 users.
But you can set 100GB quota for email if you want, with a lot of accounts.
If the average usage is 1GB, you can have 170 users.
You can even set "disk quota", which mean no limit at all.
You just have to watch from time to time if an user don't abuse of this amount of space (there is statistics).
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