Commander Zero recently linked to a source for reasonably-priced jerrycans (note that I said "reasonably priced", not "cheap"! Ain't no such thing as a cheap new jerrycan these days . . . ) In a subsequent post, he discussed why it was a good idea to have some of those containers on hand, and how much fuel he felt he needed to store.
I entirely agree with him about the need for high-quality gas cans, and have a number of jerrycans in my own reserves. Sadly, I paid higher than he did for 8 new jerrycans earlier this year - I got mine from Deutsche Optik; high-quality, but a price to match (I paid less than they're asking now, though).
As to how much fuel to store, ask yourself how many of your vehicles and appliances need fuel, and how much; then figure out how often and/or how long you'll need to use them in the absence of regular gas-station fuel (due to natural emergencies, shortages, or whatever). I have a number of calculations I use.
- I want enough to fill my vehicles' fuel tanks to the brim, plus have another tankful apiece that I can load into their cargo space. That way, if we have to evacuate for any reason, we can go a fair distance without having to worry about refilling at gas stations that may already be swamped with other evacuees, and/or may have run out of fuel.
- Over and above that, I want enough to run my generator two to three times a day, for about two hours each time, for a minimum of two weeks. That's enough to keep my freezers cold, charge essential items like laptop computers and cellphones, and do anything else for which I'm likely to need power.
- I may need gas for emergency tools like chainsaws, etc. That's not a high priority for me in this area, but if you live in a heavily wooded area where trees might come down in a disaster, plan for it.
- I may need gas for friends who need help. I can't store enough for big needs, but the ability to spare them a couple of gallons here or there might be very useful.
- I'm not worried about lawnmowers or garden appliances. In a disaster where fuel isn't available, a neat, tidy garden is likely to be the least of my worries!
There's also a legal aspect to consider. Many towns and cities have fire regulations forbidding the storage of more than a certain quantity of fuel in domestic premises, and they usually require it to be stored away from the residence (i.e. in a detached garage, a garden shed, under a tarpaulin in the back yard, whatever - but not in the same building where people live!). That's a very important consideration. Such regulations exist for a reason, and we'd be very unwise to disregard them.
So, there's a foundation for your own calculations. Work out how much you need and where you can safely store it, then buy the necessary containers to hold it. If you can afford them, I very highly recommend jerrycans - they've been a world standard since World War II, and for good reason. If you can't, the plastic containers sold at supermarkets and auto spares shops are a short-term solution, but they're nowhere near as robust, and aren't safe to carry in your vehicle, because the fumes can leak out. A jerrycan doesn't have that problem.