In the Torah (Devarim- Deuteronomy 6:9), every Jew is commanded, “Write them on the doorposts of your house and on your gates.” This commandment means that a Jew must install a mezuzah on every doorpost in their home , whether owned or rented.
Every room that has doorposts in which a Jew lives , other than the bathroom must have a mezuzah. Therefore, a Jewish home typically has numerous mezuzot in order that the house, porch, bedrooms, living room, play room, garage (if it’s used for storage and not just cars), laundry room, etc. all are properly set aside for Jewish living. Closets, and other small spaces, that are not large enough to be used (or are indeed not used) for normal living also do not have a mezuzah. The mezuzah is installed on the right doorpost as you enter the room, about 2/3 of the way up the door post. Ashkenazi Jews install it with the top tilted in to the room. Sephardic Jews have it going straight up and down.
One only says ONE blessing for all the mezuzot being put up at one time.