you can probably scrape by with a dozen 55-gal. drums per person. Apartment dwellers aren't going to be able to manage storage space for this much water unless you're a single person living in a 4-bedroom apartment! If you live on the second floor, that floor might not be up to the task of supporting the weight of twelve 55-gal. drums full, which will be over 2.5 tons!
What to do? Plan on having at least one month's supply on hand, which will cut the weight down to about 500 lbs. or one 55-gal drum (again, if you scrimp a bit). Don't forget some type of pump for your drum if you decide to go the drum route! If you get 3/4 of the way through the month and it doesn't look like the crisis is going to end any time soon, scout out a new supply. This may be a creek, river, pond or lake, or perhaps a natural spring of some sort. Don't forget to keep some type of water purification chemical and some 3-micron or finer filters on hand to filter out heat- and chemical-resistance organisms that may be present.
People who decide to draw tap water into their own containers for storage should treat the water with unscented Clorox (1/2 to 1 tsp. per 5 gal.). Re-treat the water every year or just replace it. I don't have any solid data on products like Aerobic 07; if anyone does, please pass it along. I do know that aerating your water after it's been sitting around helps restore the taste.
If you're anticipating a power outage due to impending storms, fill up all of your containers (even Dutch ovens), sinks and bathtub with tap water. When the crisis is over you can still use the water or just keep the containers filled for another time.
At the cabin I had a dug well topped with a pitcher pump. I also had several springs I could access - so many of them in fact that until the middle of summer there was always moisture in the fields around the cabin. A small creek in the back yard ran almost all year. The well was located across the road and up a rather steep grade. I acquired several of those Nalgene water containers I'm always talking about because they were spill-proof. In the winter I drove my Jeep up to the pump and then transported the water in Nalgene back to the cabin where I transferred it to covered 5-gallon buckets. In the summer I just filled the buckets and carried them down the hill since I didn't have to worry about spilled water turning my clothes or the path into icicles. When I was done pumping I always covered the pump head with a plastic bag topped with a 5-gallon bucket to keep nest-building bees and insects out. One thing that's very important with a pitcher pump is to allow the water in the pipe to drain back down into the well when you're finished. This will keep the water from freezing in the pipe in the winter.
The water was good from a bacterial contamination standpoint, but it tended to need straining because of small bits of organic material that got into the well from the rock sides. It also had a great deal of iron in it, as does most of the water in that part of upstate New York, so I had to religiously scrub my stockpots to prevent mineral buildup. Having minerals in your drinking water isn't a bad thing, since minerals that are water-soluble in your well are also absorbable by your body. Have the water tested for bacteria and heavy metal contamination before you plan on using it for your main source. The creek water got boiled before usage since I had no idea if dead animals might be lying in it upstream!
I don't live at the cabin anymore, but we do have 3 viable sandpoint wells (connected at the well pump) with (tested) potable water, plus a 55-gallon rain barrel. I purchased 2 new leathers for the pitcher pump in case we need to use it, though we can run the well pump with the generator if we're looking at prolonged power outages. We have a 19,000 gallon pool for utility water, and a 40-gallon fresh water tank in our motorhome, plus a number of 3-5 gallon Nalgene water containers, and several 1-gallon water containers.
There's more on water storage on the Water Storage page.