Mammalian red blood cells are typically round biconcave discs. Camelids are the standing exception: their erythrocytes are flattened ovals, and unusually small, which raises the total surface area available for gas exchange per unit of blood volume.
The shape also has a mechanical consequence. Camelid red cells tolerate rapid osmotic change far better than round cells do, so an animal that rehydrates by drinking a large volume at once does not destroy its own blood cells. Their haemoglobin has a high affinity for oxygen, which helps loading in thin air, and guanacos live from Patagonian sea level to Andean plateaus above four thousand metres.
Camelid blood also carries a high red cell count relative to body size. What is not cleanly separable is how much each trait, cell shape, cell size, haemoglobin affinity or count, contributes to high-altitude performance, since they evolved together and are usually measured together.