To find the ratio of the number of cups of pancake mix to cups of water, we need to compare the amounts given. The pancake mix is 12 cups, and the water is 9 cups.
We can express the ratio as:
\[ \text{Ratio} = \frac{\text{cups of pancake mix}}{\text{cups of water}} = \frac{12}{9} \]
Next, we simplify this fraction. To do this, we find the greatest common divisor (GCD) of 12 and 9, which is 3. Now we can divide both the numerator and the denominator by their GCD:
\[ \frac{12 \div 3}{9 \div 3} = \frac{4}{3} \]
Therefore, the simplified ratio of the number of cups of pancake mix to cups of water is:
\[ \boxed{4:3} \]