The first thing to do is log into your reseller portal by going to resellerportal.reachmail.net. Once you're logged in you will want to navigate to the Settings icon in the upper right hand corner.
Once you click the Settings icon you will be brought to your reseller account settings page. To set a limit on accounts being newly created with a maximum monthly volume, simply click on the bold text next to "Accounts Maximum Monthly Volume:". This will allow you to set the amount to Unlimited or a specific number. The number gets applied to new account when they register or are created manually from your reseller dashboard.
For help modifying this amount on an account-to-account basis, click here.