I am not sure there is any magic "safe" number. Theoretically the risk for spontaneous bleeding from thrombocytopenia is when the platelet count is less than 10,000. So, theoretically, anything above that should be okay but who is really comfortable starting full dose Lovenox at a platelet count of 12,000? I usually go with above 80,000. There is no science for that it seems like it should be okay!