Knowledge of the number of active bubbles in the acoustic cavitation field is crucial for predicting the performance of ultrasonic reactors in chemical processes induced by ultrasound. This chapter reviews various semi-empirical techniques developed recently to predict the number of active bubbles in sonicated aqueous solutions, with a focus on their application to sonochemical processes. H₂O₂-based techniques, hydrogen-based technique, CCl₄ pyrolysis-based technique, and thermodynamic approaches are discussed, all showing a consistent trend of increasing bubble numbers with rising ultrasound frequency. While direct comparisons between these models were not conducted, each provides valuable insights into key parameters affecting the number of active bubbles during sonication. The findings contribute to understanding the impact of ultrasound frequency on sonochemical processes and highlight the ongoing nature of research in this field, aimed at optimizing reactor performance and efficiency.