I just finished reading a great book on the Battle of the Atlantic. It said that they had trouble fielding escorts in 1939; in fact one convoy sailed unescorted until joined by one corvette and one armed trawler. From the Convoy Commodore: "Oh, look, they've sent us a rowboat and a raft!"
By 1943 you could expect to see as many as 12 destroyers, and sometimes an additional hunter/killer group patrolling close by, which meant six more destroyers and a small carrier, which meant aircraft. Not to mention blimps.
