Assignment Caps are used to limit the number of leads a User or Team will be assigned in a day from a specific lead Source. This limit does not include manually created leads or contacts added via CSV uploads.
The Cap is on a 24-hour time period for the day. To access the Assignment Caps navigate to Settings >> CRM >> Lead Management, then scroll down to Assignment Caps.
To add an Assignment Cap simply click the blue "Add an Assignment Cap" and select a User or Team, a specific Source and # of Leads to Cap them at per day, then hit to add the Rule.