Registration / Ticket type fields as well as Quantity type fields allow you to set a limit per field. This lets you enforce limited seating or selling a limited quantity of items.
To set a quantity limit, edit the registration field or quantity field of interest and set "Quantity Available" to a number greater than zero.
You can monitor the status of the quantity limits on the dashboard overview for a given event. Once the limit has been reached, a "Sold Out" indicator appears in place of the field on the form for the user to see.