When a point light source, of power W emitting   monochromatic light of wavelength $\lambda$ is kept at a distance r from a photosensitive surface of work -function $\varphi$ and area s, then number of photons striking the surface per unit time is

The number of photons released per unit time
$\mathrm{n}=\frac{\text{ power }}{\text{ energy of each photon }}=\frac{\mathrm{W}}{(\mathrm{hc}/\mathrm{\lambda })}$
These photons are spread out in all directions over an area $4{\mathrm{\pi r}}^2$. The share of an area s is $\left(\mathrm{s}/4{\mathrm{\pi r}}^2\right)$ of total  number of photons. So, the number of photons striking area s of photo-sensitive surface per unit time
${\mathrm{n}}^{\mathrm{\prime }}=\frac{\mathrm{W}}{(\mathrm{hc}/\mathrm{\lambda })}\times \frac{\mathrm{s}}{4{\mathrm{\pi r}}^2}=\frac{\mathrm{W\lambda s}}{4{\mathrm{\pi hcr}}^2}$