The number of reactant molecules which take part in the formation of activated complex in the slowest step of a chemical reaction is caleed Molecularity of that reaction. Molecularity is a whole number but can never be zero or fraction. Molecularity more than three is generally rare.
Reaction may be uni, bi or ter-molecular depending upon whether one, two or three reactant molecules are involved in the slowest step of a chemical reaction.
Molecularity of a reaction is a theoretical value. So, it is not the real quantity like order of reaction. For a complex reaction, the molecularity of reaction is expressed for each step and hence overall value is meaningless.
2 O3 = 3 O2
O3 → O2 + O (Fast) (Unimolecular)
O3 + O = 2 O2 (Slow) (Bimolecular)