Well, let's look at this logically with three questions: (sorry for the long post)
First: what disaster are you expecting and what kind of length of time are you planning to be int eh situation. (please don't say Zombie invasion - just don't)
Two: what uses do you have fro the ammo; and B) what other non-gun supplies do you have on hand?
Three: what skill level are you at, expect to be at, and will be capable of in stressful situations.
So! For number 1: Floods/hurricanes are different from war or social unrest.
For example: If you are preparing for a short-term natural disaster like Katrina, where within a few months you will be rescued and things will go to normal.
2) would be: personal defense from raiders/thugs. Maybe minor hunting. Possibly signalling, or fire-starting.
B) there is probably food around if you look in homes/abandoned stores, so hunting will not be the only source of food. Since a person can live for weeks without food - and the timeline is a few months you can see that foodwise- you may only need one big game kill, or maybe a few small varmints. Peg that at maybe a few dozen rounds.
Signalling/other uses will be negligable.
Defense may require a few more - again a few dozen for a short period of time. If many thugs or all-out raider battles are expected (not likely if a disaster is not apocalyptic) then maybe a few hundred in a main weapon and another few hundred for a sidearm.
3: your skill determines what you can do: if a big-game kill lasts for a month, and it takes 6 shots to take one down (including misses or animals that manage to escape) then you can multiply that by the months you plan to live off of hunting. Ditto with gun battles - firing hundreds of rounds is easy, but why fire if your not aiming? A warning shot can be used to scare many away, saving lots of shooting. If a fight does come down it'll be against a few thugs, take your time and aim.
As can be seen for a short-term disaster a few hundred rounds can do it. BUT changing the info in the questions changes the need! If you plan to move alot, pack less. If you are in the apocalypse fortress fighting hordes of raiders (or Zombies - yes I went there) buy an armoury!