Question

How much electrons can absorb a single photon?

Answer 1

Now these electrons could absorb the gamma ray if they have two energy levels as per QM that have a difference in energy levels that match the energy of the gamma ray. You are asking which electron will absorb the photon. It is all probabilities. Yes it is possible that both electrons will be excited by one photon.

Answer 2

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 An atom can absorb a photon, and the atom does so by raising an electron to a higher orbital. Second, the size of the photon and the size of the atom are irrelevant.