I would say that as many deer as you see, and at least an equal number detect you and avoid you.

There are plenty of times I just barely catch a glimps of a deer, and if I hadn't been looking in that exact spot and that exact moment I never would have seen it. At least twice last season I saw deer slowly creeping towards me over 80 yards away through dense cover, and then they pause for a little while and turn around and walk back the way they came. I hunt in a lot of areas where the deer move in all directions and you have to watch 360 degrees around you.