Find the number or sets of isosters

$\left({\mathrm{N}}_2,\mathrm{CO}\right),\left({\mathrm{N}}_2\mathrm{O},{\mathrm{CO}}_2\right),\left(\mathrm{HF},{\mathrm{OH}}^{-}\right),\left({\mathrm{B}}_3{\mathrm{N}}_3{\mathrm{H}}_6,{\mathrm{C}}_6{\mathrm{H}}_6\right)$

The number or sets of isosters are 4.

Any of a collection of molecules or ions that have chemical or physical properties and the same amount of valence electrons. 

All the given element i.e. $\left({\mathrm{N}}_2,\mathrm{CO}\right),\left({\mathrm{N}}_2\mathrm{O},{\mathrm{CO}}_2\right),\left(\mathrm{HF},{\mathrm{OH}}^{-}\right),\left({\mathrm{B}}_3{\mathrm{N}}_3{\mathrm{H}}_6,{\mathrm{C}}_6{\mathrm{H}}_6\right)$ has the same chemical or physical properties and the same amount of valence electrons.