The innermost electron shell of an atom can hold up to electrons.

Explanation:

The innermost electron shell is the lowest principal energy level, i.e n = 1.

For n = 1 there is only one orbital, the 1s orbital.

As stated by the Pauli's exculsion principle an orbital may have a maximum of two electrons, and they have opposed spins.

Then, the innermost electron shell has just one orbital and, in consequence, can hold up to 2 elecrons.

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