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We define the molecularity as the number of molecules taking part in a chemical reaction. It can have value one two and occasionally three. It cannot be negative, zero or fractional. It also cannot be greater than three. This is due to the fact that more than three molecules cannot collide with each other simultaneously. Even if they collide these collisions are not effective collisions which lead to the formation of a successful product. Therefore, molecularity is applicable only to an elementary reaction. It can be obtained theoretically. Molecularity and order of reaction both are equal for elementary reactions because these are single step reactions.
If we define order of reaction, it defines as the sum of powers of the concentration of the reacting species in the rate expression. It is an experimental concept and it is not necessarily related to the stoichiometric coefficients. It can have fractional value and need not to be an integer. It remains constant during the reaction.

Order of the reaction depends on temperature and pressure. So it may change depending on these factors.