The Other Ryan at Total Survivalist Blog posted an oft asked question: How Much Ammo Is Enough? Without getting into actual hard number specifics I replied with the following:
Ah, the REAL how much is enough question. :)
I look at my ammo requirements as units of training sessions, or, in harsh times, as loadouts on my various rigs.
For example, I practice pistol one to two times a month when I can. That is about 200 rounds max a month. I try to train on a consistent basis year round and having a year's worth of ammo at any given time is where my comfort level is at. So, for training pistol, I want to float around 2400 rounds at any one time (give or take 500 rounds before buying more).
I apply this idea to ammo that I carry or may use in a defensive scenario. Instead of training sessions, I think of it in terms of "per loadout". So, for example, I carry my 9c plus a reload for EDC. That's 25 rounds per loadout without reloading magazines administratively i.e. post engagement. The chances of me being accosted are fairly slim so I don't keep a lot of defensive pistol ammo on hand. I run my inventory of EDC pistol ammo at around 250 rounds and shoot some of it up every few months after I have secured more of it. This ensures fresh ammo because riding on my hip every day in humidity and less than ideal conditions with time could affect the reliability of ignition.
For a SHTF scenario, I want to have at least 3 loadouts worth of ammunition for my rifle and pistol. This means with my heavy loadout that I will be running 11 rifle mags plus 3 pistol mags.
So, while I have hard numbers for acceptable minimum ammo levels, there is a method to the madness with the only real variable being comfort level (as you mentioned in your post).
All that being said, if my other preparedness and financial goals are at acceptable levels, then I certainly try to run by the old adage, "Buy it cheap and stack it deep."
Have you given thought to this question? It's worthy of consideration when you think about the attempts around the country on the federal, state and local levels to limit ammunition sales. Do you have a plan other than "buy lots"?