My father had a derringer, a Browning 9mm (his service weapon as a cop), a pellet gun that used compressed air, a 20-gauge shotgun, a .22 rifle, two muskets, a flintlock pistol, a matchlock pistol, a pre-Civil War-era muzzle-loading rifle, and a Winchester Model 1873 that he and I were both about 90% certain was only a replica. The only ones he kept ammo for were the 9mm, the shotgun, and the pellet gun, and I don't think he bothered keeping in practice for the last 10 years or so.
Alas, my sister was executor of his estate and stole them all when she ransacked it for anything valuable, so I still own no guns myself.
One of my friends has a pistol and no other firearm. He purchased it and acquired a CPL because he wanted it for when he made his nightly cash deposits from his business at the bank.
Another friend has, I believe, a shotgun, two pistols, and at least two hunting rifles of some sort. He also has an ammunition press and whatever else is needed for reloading your own ammo. Bought it when he still bought into the yearly "Obama's taking all the ammo off the market" scares. He is, however, an avid hunter and enjoys target shooting.
Overall, I would expect the 'average' gun owner, if there is such a thing, to have between 1 and 3 guns and a box or two of ammunition.
