You can set a maximum capacity per ticket and product. This prevents you from accidentally selling too many products that you do not have in stock or exceeding the maximum number of visitors allowed according to your permit. When the maximum capacity has been reached, the product or ticket can no longer be purchased and will be marked as 'Sold Out' in the checkout. 

For tickets, it is possible to set a capacity per time slot and optionally per rate. For example, you can set that you have a maximum of 100 early-bird tickets. The general capacity of tickets is set under the 'Restrictions' section. The capacity of time slots is set under the 'Time Slots' section.

For products, you can set a capacity per product variant. This way, you can prevent selling too many shirts of a specific size that is no longer in stock. The general capacity of products is set under the 'Restrictions' section. The capacity of product variants such as shirt sizes and the like is set under the 'Product Variants' section.