You use upon between two occurrences of the same noun in order to say that there are large numbers of the thing mentioned. Row upon row of women surged forwards
upon In addition to the uses shown below, upon is used in phrasal verbs such as `come upon' and `look upon', and after some other verbs such as `decide' and `depend'
If one thing is upon another, it is on it. He set the tray upon the table He bent forward and laid a kiss softly upon her forehead = on
You use upon when mentioning an event that is followed immediately by another event. The door on the left, upon entering the church, leads to the Crypt of St Issac Upon conclusion of these studies, the patient was told that she had a severe problem. = on