Answer
$3.98\times10^{14}$ photons.
Work Step by Step
Energy of one photon
$E=\frac{hc}{\lambda}$ where $h$ is the Planck's constant, $c$ is the speed of light and $\lambda$ is the wavelength.
$\implies E=\frac{(6.626\times10^{-34}\,Js)(3.00\times10^{8}\,m/s)}{633\times10^{-9}\,m}=3.14\times10^{-19}\,J$
$\text{Total energy}=\text{Power}\times\text{time}$
$=5.00\times10^{-3}\,W\times25.0\times10^{-3}\,s=1.25\times10^{-4}\,J$
$\text{No. of photons}=\frac{\text{Total energy}}{\text{Energy of one photon}}=\frac{1.25\times10^{-4}\,J}{3.14\times10^{-19}\,J}=3.98\times10^{14}$
