You are able to set a limit to the number of tickets that can be purchased across all ticket types. For example, if you have two ticket types with 100 tickets available each but the max allocation is set to 70, customers will only be able to purchase 70 tickets out of the 200. Once the max allocation is reached, the event will automatically set to SOLD OUT.
The MAX TICKET ALLOCATION can be found under the 'ticketing' tab of the 'edit listing and tickets' section of your event admin. Once ticket types have been added, scroll down to the 'Advanced Options'.