How many secondary spermatocytes are required to form 400 million spermatozoa?

Secondary spermatocytes are formed after the meiosis I of primary spermatocytes. During the meiosis II, each secondary spermatocyte divides into two spermatids. Each spermatid then undergoes further changes to mature into a single spermatozoon (spermiogenesis). 

Number of secondary spermatocytes = Number of spermatozoa / Number of spermatids per secondary spermatocyte
= 400 million / 2
= 200 million