If you need a postbac for a career change then you need to complete prerequisites. Assuming each class and applicable lab is 4 credits you would need a minimum of 36 credits for the full year sequence of bio, phys, gen chem, orgo, and one semester of biochem. Some schools require two additional upper level bio courses which would bring this total to 44. Others have a math or statistics requirement. If you are working full-time, two classes per semester is sufficient. If you are pursuing a postbac for grade enhancement then this is different. You’re looking at taking classes to raise your GPA to prove you can handle the rigors of medical school.