Question

How many electrons does it take to fill all molecular orbitals formed from the 1s orbitals of 2 atoms?

Answer 1

There will be two molecular orbitals which will take four electrons.

Explanation:

The number of atomic orbitals will always equal the number of molecular orbitals.

Since there are two 1s atomic orbitals there will be two molecular orbitals. One of these will be a `sigma`-bonding orbital and the other will be a `sigma^"*"`-antibonding orbital.

See here for the molecular orbitals of two hydrogen 1s orbitals.