With the recent pricing changes, many resellers previously were not strictly limited by the number of accounts they could create but instead, were limited by the resources available, mainly storage. Currently, there is no way to determine from WHMCS how many accounts a reseller has and the only way to impose a limit would be to change the package for the reseller to a new one with limits.
Ideally, it would be very helpful to have a column in WHMCS under Clients > Products/Services > Reseller Accounts that displays the total number of accounts for the reseller. The information is already in WHM > Resellers > Show Reseller Accounts - under each owner, the total number of accounts is displayed there.
From a hosts' perspective that provides a large number of reseller accounts, it's important to have this value to determine which resellers have X number of accounts. Prior to the per-account billing model from cPanel, this was not really necessary.