Is there a way to restrict the number of tickets purchased for one perofrmance in an order?
Yes...I just tested it and have it working for a special offer rate that we have. You can now use the offers tab under Modes of Sale and enact a ticket limit. Originally it would only limit it on non Web MOS but in the latest version it was rolled out to the web. You will need to get some messaging code written so you actually tell the patron what is happening rather than them getting the message that something went wrong.
There is a loophole in this because if the patron goes away and creates a brand new order they can buy another lot of tickets, etc.