Why set a max tickets per purchase?
Under your venue's Advanced Options you will see a field for "Max tickets per purchase." The option to set a maximum number of tickets per purchase allows you the opportunity to work more closely with large groups that could potentially become repeat customers. When you set a maximum tickets per purchase, large groups will have to speak with you directly in order to complete their purchase. This gives you the opportunity to build a working relationship with these organizations.
Setting a maximum number of tickets per purchase can also help protect you by reducing the number of charge-backs incurred should someone make a purchase using a fraudulent credit card.
If you do not want to limit the number of tickets your customers can purchase, then insert the number you have set as your venue capacity into the field for "Max tickets per purchase."