You can indeed set a max capacity for your event to ensure you never sell over your max capacity.
The max capacity is set on a per date basis. If, for example, you have multiple dates you can set different capacities per date. You can set the max capacity when creating your dates on the Create Event form or on the Edit Dates & Times screen for existing events.
Then when creating your ticket types you can set the quantity for each ticket type knowing you will never sell over your max capacity. This is handy if you have a range of ticket types but don't want to assign a set quantity to each ticket but you are happy to sell up to your event capacity over a range of different ticket types.
For example, if you event capacity is 1000 tickets. You can create a General Admission ticket and assign 1000 tickets and create a Concession ticket and assign 1000 tickets. When you sell 1000 tickets in total across all your ticket types for your event date, all your tickets will go off sale for that date even if there are still tickets left for each ticket type as you have reached your max capacity.
You can change your max capacity at any time on the Edit Dates screen.
NOTE: Max capacity is for paid for tickets, free tickets, pay what you can and table tickets. Your max capacity does not include donation ticket types or add-ons