That details only why we would want a significant amount, not why we would want the ludicrous amount we actually have. We have 9K warheads (5k if you don't count the warheads that are intended for dismantlement). Put that into perspective that there are only like 200 countries recognized by the US. That's 25 bombs for every nation.
Russia has TWELVE THOUSAND nukes.
Source It's assinine. Even if you expected a failure rate of something huge like 60%, there's still no reason to have so many nukes.
Now, consider, any non-specialized building within 2.7 miles of smaller nukes is destroyed. Between that and 4.7 miles, any normal structure is destroyed and sky scrapers are seriously damaged. We'll ignore fallout and weather problems for the moment.
(Numbers from
Homeland Security.
So, 78.5 square miles are essentially destroyed in a modern nuclear blast. The US is 3,794,101 square miles, according to wikipedia. To blanket the entire area with nuclear blasts, it would take about 50K nukes (the current world estimate is about 30k across all nations).
But this was assuming all the bombs detonate on the ground. If they detonate in the air, within 10.7 miles all buildings are destroyed, from there to 20 miles, all but skyscrapers are. So the surface area destroyed by these blasts is 1,256 square miles. That's 3000 25 megaton atomic weapons. I don't know how many of these there are in the world. But it's feasible that the US alone has enough weapons to completely obliterate our own country, without even bothering to consider people dying from fallout or nuclear winter. We can, potentially, literally level everything.
Yeah, the potential need to hit multiple countries at once and account for failures doesn't look so pleasant now.