It seems no one has responded yet, so let me clarify it for you:
Tick the highlighted checkbox, scroll down and enable overselling. This will allow the user to create as many accounts up to the limit of 10 or until they reach their resource limit of 5GB.
The highlighted option enables quotas for bandwidth, disk space, etc for a reseller. You should, however enable overselling to allow the reseller to create accounts until the resource limit is reached.