If you are selling more than one type of ticket for a performance, tour or occurrence of your event, then setting and using capacity management will ensure you don't oversell or have to manually update your available tickets.
Once you have set a capacity you can add multiple different tickets, all with a quantity available up to the capacity number. The system will then automatically manage the sales between ticket types up to the capacity upon which tickets will then be sold out.
Capacity = 100
General Admission: 100
In this example there are 300 total tickets on sale, but the capacity is set at 100 so only 100 tickets in total will be sold.
There is more on Capacity vs Inventory here and click here for help with how you can set an event capacity