How to require multiples of 12 items on order form

I want to ensure that when ordering, the customer buys multiples of 12 bottles.  How can I set this up?  Don't see a calculation or condition for multiples.

salamone@vip.net GBwines Order Form

Thank you.

What is the next minimum orderable amount of bottles after the 12, is it a 13 or 24 value? The solutions are different depending on your answer.

Multiples of 12.  So 12, 24, etc.  What would be great would be to make the box only accept multiples of 12 but to have a little hover box that says call the winery if you have questions about this requirement.  We can actually squeeze a 13th bottle into the box so they could call to negotiate this, but I want that to be a special option, not for everyone. I want everyone to order in multiples of 12.  

I have configured it on your form. The following actions were applied.

Added two Form Calculation widget fields, first to calculate the amount of ordered bottles.

And the second to divide it by 12.

Added a Text field (you can adjust the text) from Basic Fields.

The last steps is added 'UPDATE / CALCULATE FIELD' condition. To hide the form Submit button and show the Text field if the total amount of bottles / 12 is a number with decimal point (contains dot).

Please check the form.

You are a genius, that works perfectly.  Thank you for your assistance.