That number is called a coefficient.
The coefficient tells us the number of molecules or atoms of reactants and products that are involved in the reaction and it also gives us the information about the number of moles of reactants and products.
for example we take an equation of a reaction of hydrogen and oxygen.
2H₂ + O₂→2H₂O
In this equation the number 2 with hydrogen and 2 with water are coefficients and they are showing that when there is 2 moles or molecules of hydrogen reacts with 1 molecule or mole of oxygen it will produce 2 molecules or moles of water.
