Most resellers wont use over 30-50 accts tops but always keep your plans unlimited for selling value. Nearly all of them go into it thinking they will sell hundreds so if you have limits on how many accts they can set up, you will only lose potential business.
We've had resellers rack up hundreds of sub-accounts prior to placing a limit on them, so be careful otherwise the one or two that do this, might bring down an entire server.
We place a limit on sub-accounts and use CloudLinux to contain resource usage per account.
This enables us to offer unlimited (as in un-restricted) domains per account without endangering the resources on the box.