You can set a minimum order requirement for your clients 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 "Settings" at the top right
- Scroll to the "Prints" section
- 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