What is exactly meant by a statement like "there are about 400 photons per cubic cm in certain region"? Should I mentally picture this as 400 discrete photons enclosed in that volume, each moving at speed of light in that medium (independently of one another)? Also by one photon I should not mean something like a particle (localized at a point) but a changing electric and magnetic field in phase spread over exactly one full wavelength. If I were able to take a snapshot of the changing fields, these would be like sinusoidal variations in space and if I knew the velocity I would know that after one second the entire field variation will be shifted exactly "c" distance away from the previous location of wave (along the wave vector) and effects of field variation at previous location is obliterated. To these one may add characteristics of polarizations etc but is the above picture basically correct?
Also, is there any restriction on the number of photons in such a picture (if it is correct at all)? Or is there any condition so that we must consider multiple photons (so that we get continous wave) rather than a single photon?