Mg + 2HCl → MgCl2 + H2

How many molecules

The total number of molecules taking pert in the given reaction is 3.

Explanation:

There are two reactant molecules participating in the given reaction

Mg+2HCL=MgCl2+H2

Here 1 molecule of magnesium (Mg) and 2 molecules of Hydrochloric acid (HCl) undergoing chemical reaction to generate magnesium chloride (MgCl2) and hydrogen gas (H2).

As a result the molecularity of the reaction is 3.