You can set a minimum order requirement for your galleries in the Gallery Settings. Your client will see a warning on the checkout page that a smaller order cannot be completed because the order doesn't meet the minimum requirement for that particular gallery.
To assign a minimum order for a gallery:
- Go to Photos > Galleries > Click on the Gallery
- Click Gallery Settings to the left, under the gallery cover image
- Go to the Shopping Cart tab
- Enable the Shopping Cart and select Traditional Store*
- Type the amount you want to set as the minimum in the Minimum Order field
- Save your changes on the top right of the page
* Order minimums can only be set in galleries using the Traditional Store. You can learn more about the print fulfillment options here.