1,456,862 personnel are currently fighting (ranked 2nd in the world).
1,458,500 personnel are in reserve (ranked 7th in the world).
"ARMED FORCES STRENGTH FIGURES FOR SEPTEMBER 30, 2011". United States Office of the Under Secretary of Defense for Acquisition, Technology, and Logistics. September 2011.
"S.3001 - National Defense Authorization Act for Fiscal Year 2009". United States Government Printing Office. 12 May 2008.
over 20,000 troops
65
US= 4,191 UK=176 Other= 138 Total= 4,505
In total, there was 156,000 Allied troops which were involved with D-Day. This included 15,000 Canadians, 60,000 British and 75,000 US troops.
40,000,000 troops
There are currently about 48,000 US troops in Japan
There are approximately 68,000 US Troops stationed in Germany.
In total 110,000 US military personnel were directly involved in the battle
The total number of troops supplied by those nations was well over 7 million, on the low end. Probably more.
The answer is not approximate but around 1,500,000 fought.
Today there are a Total of 2,475,967 soldiers in the Military. There is 507,158 soldiers in the army, 347,693 soldiers in the navy, 347,352 soldiers in the air force, 179,762 marines, and a total of 1,381,965 DODs
http://www.defenselink.mil/news/casualty.pdf