How many boys give their left hand to a girl?

In such arrangement, the boys and girls hold their hands (right hand to other's left hand) or (left hand to other's right hand).

Exactly, $18$ boys' right hands are holding 18 girls left hands

$\implies 40-18=22$ boys' right hands are holding $\textcolor{#3D99F6}{22 boys'}$ left hands

$\implies \textcolor{#3D99F6}{22 boys'}$ left hands are holding $22$ boys' right hands

$\implies 40-22=18$ boys' left hands are holding girls.