This is due to the following reasons:

* Quantum Numbers: The innermost energy level corresponds to the principal quantum number (n) = 1. This level only has one subshell (s), which can hold a maximum of two electrons.

* Pauli Exclusion Principle: This principle states that no two electrons in an atom can have the same set of four quantum numbers (n, l, m_l, m_s). The "s" subshell has only one orbital, which can hold two electrons with opposite spins.

Therefore, the innermost energy level can only accommodate two electrons with opposite spins.