human biology - How do people who have lost both of their legs produce red blood cells?

Red blood cells are produced in the red marrow which...

"is found mainly in the flat bones, such as the pelvis, sternum,
cranium, ribs, vertebrae and scapulae, and in the cancellous
("spongy") material at the epiphyseal ends of long bones such as the
femur and humerus." - Wikipedia

So you are partly right; the femur is associated with red blood cell production, or Erythropoiesis to give it it's technical name, but there are other bones within the human body that also do this job. The process of erythropoiesis is stimulated when the kidneys detect low levels of oxygen in the blood stream and stimulate production of the hormone erythropoietin. Further, the role of the tibia and femur in erythropoiesis also decreases with age whereas...

"the vertebrae, sternum, pelvis and ribs, and cranial bones continue
to produce red blood cells throughout life." - again from the wiki page

So I'd suggest it is unlikely that loss of the legs would have a major impact on the production of red blood cells in adults. I imagine that with the loss of legs comes some reduction in functionality of erythropoiesis but also a lower requirement of red blood cell production (less blood capacity = less blood cells needed = less blood cells need to be produced). I can't find any studies which explore the ability or needs of amputees and non-amputees with regards to red blood cell production.