Interesting property of dihedral group

Question

I know the number of normal subgroup in $D_3$ and $D_4$ but I don't know the number of normal subgroup of rest of the Dihedral group like $D_{16}$ or $D_{60}$. I want to know is there any formula for finding number of normal subgroup in $D_n$. If there is no formula for that then how could we able to find that as I'm a beginner please help.