The hard limit would be when you get a "out of disk space" errors.
The amount that comes with your plan is at the bottom in the
"Disk Usage by Main Plans (sub-plans usage count on main plan)" section,
where it does the math for you.
Each ftp/shell/mailbox user takes up disk space. And since you can have multiple users on a single plan, each user will share a portion of the plan total disk space.
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