Being an old editor, if a reporter asked me this question, I would tell him to look it up from a reliable source.
Like msorensson said it depends on sex, height, weight and other factors. The numbers range from as little as 3.5 liters for a woman to 7 liters for a man. The avearage seems to be around five. Do a Google search and you can found out everything you want to know and more about the body's blood capacity.
By the way, if you are in good health, give consideration to donating blood to your local blood bank. Donated blood is truly the gift of life. I give regularly. Had to stop for awhile while I was adjusting to some new medicine but now my doctor has given me the go ahead and is encouraging it.