To solve for the missing number in the expression \( \frac{7^x}{7^5} = 7^{12} \), we can use the property of exponents that states:
\[ \frac{a^m}{a^n} = a^{m-n} \]
Applying this property to our expression, we have:
\[ \frac{7^x}{7^5} = 7^{x-5} \]
So we can rewrite the equation as:
\[ 7^{x-5} = 7^{12} \]
Since the bases are the same (7), we can set the exponents equal to each other:
\[ x - 5 = 12 \]
Now, we can solve for \( x \):
\[ x = 12 + 5 = 17 \]
Thus, the missing number in the expression is:
\[ \boxed{17} \]