Is each Mezuzah a separate Mitzvah?

Question

A house needs to have Mezuzos on every doorway that is an entrance to an appropriate room. The question I have is: is each Mezuzah a separate Mitzvah on it's own, or do they all combine to one 'larger' Mitzvah of having a properly Mezuzahed house?

Possible Nafkah Minahs:

• You have a house with 10 doorways that should have Mezuzahs, but only have 8 Mezuzahs. If each one is a separate Mitzvah, then you would likely put up what you have. If it's one larger Mitzvah, maybe there is no need to.
• You are doing construction on your house, and have the option of constructing certain doorways either in ways that obligate one it in a Mezuzah, or in ways that exempt it from a Mezuzah. If each doorway is a separate Mitzvah, maybe one should make as many doorways as possible obligated in Mezuzah.
• You don't have time to put up all the Mezuzos for your house at one time. Instead, you put them up on two separate occasions. If they're each separate Mitzvos, each distinct time you put up Mezuzos requires a new Bracha. If it's all one Mitzvah, perhaps you should only say the Bracha before the last batch that you put up.

Answer 1

I have worked as a Sofer Stam for many years and learned and researched these issues, both for my customers' sakes and for my own knowledge. Here is what I have discovered:

1. Each room is its own mitzva. The Talmud and the Rambam use the term בית which would usually indicate an entire dwelling rather than a room, but during the time of the Mishna most dwellings were single rooms. Also, the Rambam in Hilchot Mezuza 6:10 refers to rooms within a house explicitly, and all the later sources clearly interpret the Talmud the same way.
2. Not all rooms have an equal obligation to have a mezuza affixed to their threshold. Since the verse specifies "ביתך", i.e. a dwelling space, bedrooms have the greatest importance according to many authorities. Storage spaces, office spaces, etc. have secondary importance, and may not even get a bracha when putting a mezuza up on their thresholds alone.
3. Each mezuza being affixed in a room that's truly obligated in the mitzva should have a bracha before it, however it's recommended to put up all the mezuzas in the batch with a single bracha, provided there can all be done in the same act (i.e. no היסח דעת).

Therefore, you should always put up as many mezuzas as you have, immediately (if you're outside of Israel you have a 30 day grace period). I've never any discussion about constructing doorways to favor more or less mezuzas. While there are people I know who want to propose that when you build a house you should do it for the mezuzas, i.e. anything you do you should look for the opportunity to get a mitzva out of it, IMHO that's totally missing the point of yiddishkeit. But that's well beyond the scope of this question. As far as the bracha, as stated above, every time you put up a mezuza (unless it's a questionable space, like a walk-in closet) you need to say a bracha.

On a related note, when switching mezuzas without any significant "down time", there are three different opinions as to whether not you say a bracha: yes, no, and "it depends"... Further details upon request.