From your control click on the Events tab.
Select the event that you would like to change the ticket quantities.
Select Edit event and tickets from the left hand menu.
Under Create tickets select the ticket you would like to change the quantity for, or click Add a new ticket type if you would like to create a new ticket type.
The Quantity field is the limit of the number of tickets it will allow to be sold, excluding any that have been sold.
If you would like to set a maximum quantity across a number of ticket types then click here to learn about how to use ticket groups to do this.