Erythrocytes contain many hemoglobin proteins. Each protein has 4 haem groups, and oxygen molecules bind to in porrin ring inside these haem groups. The binding capacity is the affinity each haem group has for oxygen. This changes depending on the external conditions, for example PH.
For example, when the ph is quite low (acidic), this is often due to CO2 build up in the local area, dissolving in the blood to form carbonic acid. This means that there is anaerobic activity happening in the area so it needs more oxygen. The oxygen associated with the hemoglobin will disassociate making it available for the surrounding tissue. It is important to remember that the process is not concrete and is always fluctuating. There are many curves that show the likelihood of association and disassociation. I would recommend looking at these disassociation curves to understand.
Erythrocytes are typically oxygenated in the lungs (supplied by the pulmonary artery from the heart). After oxygenation, the blood goes back to the heart via the pulmonary vein in order to be sent round the body. Reticulocytes are immature erythrocytes and will be carrying increased amounts of haemoglobin as they develop it. The following link leads to a paper which approximates haemoglobin content in reticulocytes.