The $\mathrm{Cl}-\mathrm{O}$ bond order in perchlorate ion

The total number of bonds is 7 . Or we can calculate it by counting the number of resonance structures this molecule can make.

$\mathrm{Bond} \mathrm{order} \mathrm{of} \mathrm{Cl}-\mathrm{O}=\frac{\mathrm{total} \mathrm{number} \mathrm{of} \mathrm{bonds} \mathrm{formed}}{\mathrm{total} \mathrm{number} \mathrm{of} \mathrm{sigma} \mathrm{bond}}=\frac{7}{4}=1.75$

So, from the above statements we can conclude that the bond order of perchlorate ion is 1.75.