However, it is misleading to speak about Catholic deaths the way we speak about Jewish deaths.  Except for parts of Poland, Catholics were not targeted as Catholics the way the Jews were targeted as Jews.  In parts of Poland the Catholic clergy was rounded up and put in concentration camps as part of Hitler's larger program to exterminate, Germanize, deport and enslave the Polish population.  Hitler had it in for the Catholic Church and intended after the war to destroy it in the areas which he controlled.  However, with 40% of Germany Catholic he could not afford to move against the Church too violently.  His persecution was limited to closing Catholic schools, publications, associations, and placing in concentration camps those who spoke out against him.

So my short answer would be that around 9 to 10 million Catholics were killed during World War II either as soldiers or as civilian populations – but that except for a few exceptions in Poland, they were not killed because they were Catholics, but rather because they were soldiers or civilians who happened to be in the wrong place at the wrong time during the war.