Some figure came out of somewhere recently (eminently believable, but I don't know the source) that the US has passed 1.1 guns per capita.  This is cause for rending of clothes and gnashing of teeth among a certain segment, which is fantastic.

The correct minimum number of guns for the US is 1.25 billion.

Let us assume 250 million adults.

EVERY adult should have a shotgun.  They are cheap, reliable, the ammo is inexpensive, and with several different loads on hand it is versatile, able to hunt anything from quail and squirrel to deer and elk.  With the right loads, they stop home invaders dead in a puddle of goo. There are custom mods you can make so it can take any of the Big Five game in Africa.  https://thefiringline.com/forums/showthread.php?t=197214 

Next, one should acquire a sidearm of choice, for portability, use in close quarters, and personal defense. Along with this, you need a similar framed .22 for less expensive practice (and hopefully again soon, cheap practice, when idiots and greedy assholes stop hoarding).

A lot of urbanites likely don't need a rifle--the shotun will cover what they need.  Still, it's worth having one just in case.  And, to go with your primarly rifle, you need another .22 for additional practice.

That's five, per adult.  

There should be two bricks of .22, about 200 shotgun shells, and at least a crate (500-1000 rounds) for the sidearm and rifle.

Given that shooting should be taught as early as a child can understand the basic safety rules (typically between 4 and 8 ), then there's room for more, and of course, different types of shooting require different types of weapons.

350 million is a start, but only a start. Americans should possess at least 1.25 billion firearms, likely closer to 2 billion, and at least 1200 rounds of ammo on hand in between range trips.

Only a coward with a small penis would argue for less.