As an event receives bookings, the number of available tickets against each ticket type will slowly decrease. Once a particular ticket hits 0, it will no longer be selectable for booking.
To add more spaces to an event:
Open the event in Manager.
Click the Add tickets tab.
Find the ticket type you want to update.
In the Qty field, enter the new total number of tickets available for this ticket type.
Click Update to save your changes.