What is the order of the reaction of a single collison?
When we speak of the order of a reaction or a step in a reaction mechanism, we are referring to the number of molecules that collide in causing that step to occur.
In this case, since two molecules of HI collide, the order of the reaction is two.
Note that single collisions involving more than two molecules are extremely rare. In proposing reaction mechanisms, chemists try to use steps with order of two to explain the mechanism of reactions that involve larger numbers of molecules as reactants.