Enabling this feature will rotate the accounts and only run a MAX amount at a time set by the user.
For example, if you have 50 Gmail accounts, and you have 25 as the Max Accounts Running At Once, only 25 will be farmed at the same time. The Gmails will be rotated and others will begin farming when the current ones running are taking a break or sleeping.
This setting was introduced to further control the CPU and Memory usage on your computer.
You will have to navigate using the arrows to get to this setting.