When you are creating a ticketed event you can choose to set a limit on the number of tickets that can be purchased by:
  1. Go to Content > Events > Next to the event in question click "Edit".
  2. Go to step 6, "Configure Ticket Types"
  3. Next to the ticket type in question click on "Edit"
  4. On Option "6. Limit Per Order" will need to be changed to "Limited".
  5. You will then select the amount of tickets that can be purchased of this type.