Did they walk around all day, go to services, dropped off food and gifts to their shrines? Did they have small personal shrines or totems in their home for the gods like we do with crosses?
Could you clarify your question? I am not sure if you are asking how many people in Ancient Greece or Rome practice religions that would in their time be dead or whether you are asking how many people in Ancient Greece or Rome practiced Greek or Roman religions and what those practices would be?