Yep....steroids and epo are very similar in that regard...any hormone is. The number of hormone receptor sites vary as does the affinity the receptor has for the hormone.
A good summary:
http://en.wikipedia.org/wiki/Hormone
"An important consideration, dictating the level at which cellular signal transduction pathways are activated in response to a hormonal signal, is the effective concentration of hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:
1.The number of hormone molecules available for complex formation
2.The number of receptor molecules available for complex formation
3.The binding affinity between hormone and receptor"
Alan