Can the body adapt to to many red blood cells?

Answer 1

Actually, no. And this used to be a common thing among athletes. They used to do what is known as "blood doping". They take out red blood cells, store them for a while (meanwhile your body has already made up for the lost red blood cells) and then inject it back to their body. They do this because it gives them more stamina. But as a result, your blood pressure increases, and your heart has to do a lot of work to pump all of that blood, so eventually you get a stroke and die. The athletes used to drop dead mid-race.