Here is an excellent site for statistics on the number dead during conflicts of the twentieth century.
http://users.erols.com/mwhite28/warstats.htm
http://users.erols.com/mwhite28/warstat1.htm
On this page you will find detailed statistics about the deaths of every (practically every) country that was involved during the Second World War.
The really good thing about this site is that it lists its sources (and its sources are reliable). It also lists different statistics from different sources relating to the same event. If you are ever doing a report on the Second World War and you need information like this, I suggest you go here.
Here is an interesting page where it graphically displays the deaths by country in different theatres. http://users.erols.com/mwhite28/ww2-loss.htm
And here lists Holocaust deaths by Nation. http://users.erols.com/mwhite28/ww2stats.htm#Holocaust
Throughout the website you can also find other interesting lists like the list of the bloodiest battles and massacres of the 20th century.