Estimating the following number should be interesting. The number tells you why our eye can never 'count photons', even in barely detectable light.
The number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can perceive `(~10^(-10) W. m^(-2))`. Take the area of the pupil to be about `0.4 cm^(2)` and the average frequency of the white light to be about ` 6xx10^(14)` Hz.

Explanation

Minimum intensity, `I = 10^(-10) W.m^(-2)`
Area of pupil `= 0.4 cm^(2) = 0.4 xx 10^(-4) m^(2)`
Average frequency, `f = 6 xx 10^(4) Hz`
`:. E = hf = 6.626 xx 10^(-34) xx 6 xx 10^(14)`
`= 3.98 xx 10^(-19)J`
`:.` Number of photons incident on eye,
`n = (IA)/(E) = (10^(-10) xx 0.4 xx 10^(-4))/(3.98 xx 10^(-19)) = 1.01 xx 10^(4)`
This is quite a small number, but still large enough to be counted.