Over 1,600,000 American servicemen and women were in Great Britain as the invasion was launched. Numbers dwindled thereafter, but as late as April 30, 1945 there were still over 224,000 airmen, 109,000 communications zone troops, and 100,000 in hospitals or preparing to serve as individual replacements.
https://www.abmc.gov/news-events/news/americans-great-britain-world-war-ii-online-interactive-released