Assuming you are referring to HVAC ductwork, the answer is not as simple as a specific CFM for a 12x12 size.
The reason for this is that the required CFM (Cubic Feet per Minute) for any given space is dependent on a variety of factors. These include the total square footage of the space, ceiling height, insulation levels, number of windows and doors, and airtightness of the home.
A general rule of thumb is that you need 1 CFM for every square foot of living space in your home. So, if your home is 1200 square feet, you would need 1200 CFM. However, this number can increase or decrease based on the other factors mentioned above.
If you want to get a more accurate estimate for your home specifically, it's best to consult with a professional HVAC contractor who can take all of these factors into account.