Oxygen gas is always written as O_2 in chemical equations. What term is used to describe the "2" in the formula?

The 2 in the formula $\text{O"_2}$ is a subscript.
The formula for a molecule of oxygen gas is $\text{O"_2}$. It is a diatomic element, meaning it is composed of two atoms of oxygen, covalently bonded. The subscript 2 means that 2 atoms of oxygen are in one molecule of $\text{O"_2}$.