I want to know that what should be the minimum level of haemoglobin to start the methotrexate?
There is no minimum level. The critical question is why is the hemoglobin low (anemia). Methotrexate at the doses used for RA rarely is the cause of anemia. However, if the reason for the anemia is bone marrow failure, then I would be concerned about methotrexate use. Inflammation can cause anemia and thus in those instances, methotrexate might actually help.