Relation between numerical value (N) of a physical quantity and its magnitude of unit (U) is                     

We can have the relation between numerical value (N) and magnitude of unit (U) is $\text{N\hspace{0.17em}}\propto \frac{\text{1}}{\text{U}}$ , because $\text{P\hspace{0.17em}}=\text{\hspace{0.17em}NU}$ 

Since P is independent of choice of unit

We get $\text{N\hspace{0.17em}}\propto \frac{\text{1}}{\text{U}}$