2+ or 2 by 2 rule

To assure progressive overload, we used the “+2 principle.” This principle implies that the patients are told to perform as many repetitions as they can manage in the last of the third or fourth sets. If they are able to add 2 extra repetitions, load will be increased in the next treatment session.

(Kraemer WJ, Adams K, Cafarelli E, et al. American College of Sports Medicine position stand. Progression models in resistance training for healthy adults)

The above mentioned criteria can be followed as a guideline