A point source of light of power P and wavelength $\lambda$  is emitting light in all directions. The number of photons present in a spherical region of radius ‘r’ to radius 2r with centre at source, is

Intensity at a distance x from point source is given by  $I=\frac{P}{4\pi x^2}$  

consider an elemental shell of radius x and thickness dx, in this

region, the energy contained is $dE=\frac{I}{c}\times 4\pi x^{2}dx=\frac{Pdx}{c}$  

Let dn be the number of photons in this elemental shell, then

 $dn\times \frac{hc}{\lambda }=dE=\frac{Pdx}{c}$

          $\Rightarrow dn=\frac{P\lambda dx}{h{c}^2}$

Total no.of photons in the shell of inner radius r and outer radius 2r is

$n={\displaystyle \int dn}={\displaystyle \underset{r}{\overset{2r}{\int }}\frac{P\lambda dx}{h{c}^2}=\frac{P\lambda r}{h{c}^2}}$