In a majority of angiosperm for the development of female gametophyte, the functional megaspore undergoes how many meiotic and mitotic divisions?

The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. 
Only during the formation of megaspores from megaspore mother cell meiosis is involved.