A box of 20 would likely not make any difference. Imagine trying to avoid catching the run of the mill cold this whole winter with just a box of 20. Remember, pandemics run for many weeks and months at a time. Well, it theoretically might work if you could stay home the entire time except for 20 quick trips into public, but how practical is that for most folks?

For the most part, I would argue that practically speaking, a respirator like an N95 provides minimal protection for your average person who isn't trained, fitted, and following a strict protocol on their use (including the proper removal and disposal of used masks). The reason that N95 masks don't really come with any instructions is because they are intended to be used as part of a whole fitting and training program provided as part of your work. That's why I listed those masks approved for use by the general public in my earlier reply.

There are just too many ways to mess it up and get exposed and for your average person who doesn't properly fit it and is just going to reuse a mask for too long until it's too soft to provide a good face seal or too moist to filter properly, etc. it's probably going to provide a false sense of security. If you're already a healthcare or lab worker and are trained and accustomed on the proper use of the respirators and change them often enough and combine them gloves, disposable gowns, etc., then the protection is higher, but even that is not foolproof.

Some may think that they'll just wear them in the highest risk situations so they don't need as many masks, like only when they're riding public transit to and from work. Statistically speaking, I suppose that reduces your chances of getting sick, but probably only just, I would think. Plenty of people at work could still infect you.

A half or full-face respirator with replaceable filter cartridges provides a much better face seal and protection, but the mask and a good supply of filters is not cheap, and that's not something that you can fold up small and tuck into your EDC kit. I don't know the shelf life of the cartridges, but none of them have an indefinite shelf life.

One alternative approach is to focus on preventing the sick people from infecting you, such as a sick family member you are taking care of. Surgical procedure masks--those flimsy paper masks surgeons or your dental hygienist wears--helps catch the tiny saliva aerosols a sick person emits when they cough or even from talking. These masks are cheap.

However, even this approach is far from foolproof. The incubation period of, say, influenza typically ranges from 1-4 days (from exposure to onset of symptoms) and you can infect others starting a day before you show symptoms. So, an infected person could pass it on to you before they even know they're sick and start wearing a procedure mask.

Assuming a raging pandemic bug doesn't burn itself out sooner, I would say you need at least a 6-9 month supply if you're going to try and rely on these masks. The idea is that you're basically trying to hold out until a crash program to develop a vaccine can be brought online. As you saw in the movie, first you have to figure out how to find a vaccine, and even if you skip the proper chain of research for safety and efficacy and start production right away, it takes many months to produce the vaccine in quantity.

Actually, during the H1N1 swine flu pandemic, it was remarkable that we had vaccine available in the same year. That was quite a feat.