There are a total of four electrons that are shared in a double covalent bond. This is because there are two electron pairs interacting in the bond.
An oxygen molecule is a simple example of a double covalent bond. In an oxygen molecule, each oxygen atom has six electrons in the outer shell; this leaves two electrons per atom needed to complete the octet. The oxygen atoms share the four electrons. A double covalent bond is sometimes written with a double short line between the two atoms, as opposed to a single covalent bond that is denoted by a single short line.