1. A method of identifying a number of people in a monitored area comprising the steps of: collecting a first reflected ultrasonic wave map of the monitored area; collecting a second reflected ultrasonic wave map of the monitored area; subtracting the first map from the second map to obtain a resulting map; processing the resulting map to identify clusters having a minimum intensity; calculating a weight coefficient of the identified clusters; and comparing the calculated weight coefficients with predefined values.