Light comes in discrete energy quanta; a monochromatic light source with frequency f will come as one hf photon, two hf photons, three hf photons, and so forth

Light comes in discrete energy quanta; a monochromatic light source with frequency f will come as one hf photon, two hf photons, three hf photons, and so forth because of the relationship E=hf, where E is the energy and h is Planck's constant. Multicolour light will come in different frequencies, but for each frequency there will be an integer number of photons. You will never see a photon with energy 1/2 hf or 3/4 hf, where f is that photon's frequency. Frequency itself is not quantized; in the visible spectrum photon frequency can take on any real value between about 430 and 770 THz. Moreover, frequency observer-dependent. Two observers related by a Lorentz boost (i.e., moving uniformly such that there is a constant relative velocity between them) will see the same photon with a different frequency (and thus by E=hf, a different energy); this is the relativistic red shift (or blue shift).