We won't be the first ones to point out that if you don't have enough ammo, your guns aren't any better than a club or shovel. They might even be less useful without ammo!
Whatever weapons you have, the general minimum rule is, store at least 1,000 rounds per caliber and have another 1,000 rounds available for practicing. Many people store more, or have goals of building up their ammo stash as high as possible.
That said, ammo is expensive and the large caliber ammunition for big weapons takes up a lot of space. Weapons experts advise that you should have at least three to five magazines for each weapon you have with a magazine. You won't have time in a genuine emergency to fool around with loose bullets. Or, picture it this way. Did you ever watch the bumbling deputy Barney Fife on the old Andy Griffith Show? Sheriff Andy gave Barney an unloaded revolver and one bullet "just in case," because there was no way skinny, hysterical Barney was ever going to carry a loaded gun around without shooting himself in the foot. For our purposes of defense, Barney's single bullet isn't going to do the job.