There is a summary here at enotes that explains broadly who is speaking and what they are saying.  The whole of Book 12 is a conversation between Adam and the Archangel Michael.  Most of the time Michael is speaking, instructing Adam, and occasionally Adam interjects or asks a question.  Essentially Michael gives to Adam the whole of human history from Noah, through the Tower of Babel, to Abraham and the founding of the people of Israel, to Moses, and finally to Christ and the redemption of mankind.  Generally, when a verse paragraph ends and the speaker changes, Milton puts at the head of the next verse paragraph something like "He [Michael] ended; and thus Adam last replied:" (12.552).  Even though there are various stories and people in this book, there are really only two speakers during most of it.  At the very end of the book Michael and Adam go and speak to Eve, but she only appears in the last fifty lines of the 650-line book.