At very high altitudes, the human body is exposed to lower levels of oxygen. This lack of oxygen triggers a physiological response in the body to compensate for the lower oxygen levels. One of these responses is an increase in the production of red blood cells (RBCs) by the bone marrow.

RBCs are responsible for carrying oxygen to the body`s tissues. By increasing the number of RBCs, the body can transport more oxygen, which helps to counteract the reduced oxygen levels at high altitudes. This increase in the number of RBCs is an adaptive response to improve oxygen delivery and maintain normal bodily functions.

Therefore, the correct option is 3: increase in number. The size of RBCs remains relatively constant, but their number increases at high altitudes to compensate for the reduced oxygen availability.