The Lewis structure for ethylene, C2H4, is shown. How many valence electrons do the two carbon atoms in the molecule share with each other in the double bond?

So,

The Lewis structure for ethylene is two carbons double-bonded to each other, and two hydrogens single-bonded to each carbon atom.

Each bond symbolizes two shared valence electrons. Since the carbon atoms are double-bonded, they share four (4) valence electrons (the other electrons are in lower energy levels and do not appear because they typically don't react when higher energy-level electrons are on top of them).