Plan's answer is better than this one, but just in case it misses anything, this is a more basic view, and I can learn myself from any comments on this.
I seem to get a higher number of physics sources saying the amplitude depends on how many photons you have and if you have one the question has no meaning. Of course I have not idea if this is accurate or how one would even test this.
I have never myself ever seen this linkage between particles and amplitudes, (although it may depend on the sophistication of the text).
The advantage of the wave/ particle is that it explains observations such as interference, if we treat electromagnetic radiation as waves, and the photoelectric effect, if we treat em radiation as photons.
Neither interpretation is more meaningful than the other, they are both methods that are useful in explaining particular observations that cannot be easily understood using a mental picture that is not appropriate to a particular experiment.
Behind both photons and waves is a mathematical framework (which is the real explanation) that allows us to calculate and predict physical results and effects in terms of photons or waves.
Normally, it's the energy and momentum associated with a physical event that we are concerned about.
If you use photons, you don't have to worry about amplitudes, energy is calculated using Photon Energy, and this reference gives you the math.
But amplitude is more a wave related concept than any application to particles.
If you use waves to describe electromagnetic radiation, the energy is given by this reference, Energy of an electromagnetic wave, which will tell you that the energy carried by a wave is proportional to its amplitude. Again, the math behind the wave picture is included.