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