When a positron and an electron come together they annihilate and produce 2 gamma photons $$e^+ +e^- \rightarrow 2\gamma$$
I can understand that they must be produced in pairs to conserve momentum. However, that still leaves all other even numbers. Why 2 photons, why not 4,6, 8, 10... photons?
Note it is NOT a duplicate of Why are two photons created in annihilation? because that question only asks how can mass-less particles have momentum.
