18.5.

$$ \frac{7^4 \cdot 9^6}{63^4} = \frac{7^4 \cdot (3^2)^6}{(7 \cdot 3^2)^4} = \frac{7^4 \cdot 3^{12}}{7^4 \cdot (3^2)^4} = \frac{7^4 \cdot 3^{12}}{7^4 \cdot 3^8} = 3^{12-8} = 3^4 = 81 $$