Calculating order frequency by weeks

Hi,

I have been trying to find the right formula for this this question:

How to count the amount of weeks an order is placed for a specific product.

For example if Product XYZ is ordered 12 out of the 52 weeks in the year, I would want my total to show 12.

I'm currently using an orders table (order number), product table (product ID) and a date table (order week) to achieve this.

Any help on which formula to use is greatly appreciated! Thanks!