The number placed in front of a compound to balance a chemical reaction is called

The number placed in front of a compound to balance a chemical reaction is called coefficient.

Explanation:

Assume this general form for a chemical equation:

The letters a, b, c, and d, in front of each compound A, B, C, and D, are called coefficients and indicate the number of formula units (molecules or ions) that take part in the equation.

Those coefficients are needed to balance the equation and ensure compliance with the law of mass conservation.

This example shows it:

        H₂ (g) + 2O₂(g) → 2H₂O(g)

In that equation:

You read it as: 1 mole of gaseous hydrogen and 2 moles of gaseous oxygen yield 2 moles of water vapor.

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