Uboat.net has a month-by-month page showing losses.
http://www.uboat.net/allies/merchants/losses_year.html
The total number of ships (including warships) hit by uboats was 2791, and that includes ships that were torpedoed or shelled but survived.
http://www.uboat.net/allies/merchants/index.html
Since the US alone built more than 2700 Liberty ships in just two years, it would seem that Germany's best chance to win the Battle of the Atlantic was in 1940. After that it was all downhill for them.
The Battle Of The Atlantic, by Terry Hughes and John Costello, has month-by-month charts showing Allied shipping at sea, number of ships and tonnage sunk, and u-boats lost.