Answer:
Figures are not specific, but can be speculated upon.
While six million Jews were killed, at least another six million were also killed. Gypsies, Soviet prisoners, Jehovah's Witnesses, Trade Unionists, Catholics, Communists, Homosexuals and political agitators.
It is certain that many of that number were Christian to some degree, that being the dominant religion of all the areas. It is just as certain that some were not, but in the balance, most were probably Christian.
If six million non-Jews were killed, and one out of six were Christian (low estimate) then one milllion Christians were killed in the Holocaust. If all were, then six million were killed, maybe more. (Some estimates have an additional 10 million non-Jews killed.)
"Several million" might be the best answer. That can be said with reasonable certainty.