The number of divisors of 18 is

If $N=P_1^{n_1}\cdot P_2^{n_2}\cdot P_3^{n_2},\dots .$then number of divisors of $N,d\left(N\right)=\left(n_1+1\right)\left(n_2+1\right)\left(n_3+1\right)\dots ..$

Number of divisors of $18=2\times 3\times 3$

$18=2^1\times 3^2$

$d\left(18\right)=(1+1)(2+1)=2\times 3=6$