At PreppGroup, Milan Adams discusses FEMA estimates for the survivability of a nuclear attack using 6,139 warheads. He writes:

Here is a 1990 FEMA prediction, the last one FEMA made during the Cold War.

Could parts of the USA survive a nuclear war?

Not just ‘could,’ parts of the country would. Allen E Hall’s answer shows the destructive capability from Russia’s current arsenal.

In 1990, a FEMA analysis of a 6,139 nuclear strike on the United States (about 3 times Russia’s current arsenal) had more than half of the country’s population not only experiencing less than a fatal shockwave, but experiencing low radiation fallout.

I am copying part of my reiteration of the report from here:

Brian Collins’s answer to What would the world be like after a nuclear war?

Here are screenshots from the report. About half the of the US had a medium to very high fallout risk:

So basically, half of the US population would need to take shelter for a few days to 2 weeks, or risk radiation exposure in a 6,000 warhead strike.

Read Adams’s entire analysis here.