When you create a new event or edit an existing one you may set the limit on the number of tickets that can be purchased/received by one customer for each showing. This works for both free and paid events.
On the event configuration page, activate the Limit ticket purchase per customer option.
Then enter the maximum number of tickets (per show) that you wish to allow for one customer and save the event settings.
When this is done, customers will not be able to purchase more tickets than this setting allows even if they create multiple orders for the same event.
However, customers may be able to overwrite this limit by using different buyer accounts on Stage Stubs. There is no possibility to prevent this from happening at this time.