One million. UN Troops are in place to do a coup against the US

An estimate of firearms in civilian possession in the US 393,347,000

An estimate of armed civilians in the US 80,000,000

The equates to 4-6 firearms per civilian

There is no estimate on ammunition lets say 1/8 are highly proficient or better, 1/4 are moderately proficient, 1/4 are low proficient, and the remaining would need to be in close proximity to be accurate

#--------- Proficiency
10,000,000 High or better
20,000,000 Moderate
20,000,000 Low
30,000,000 Poor

The estimate above does not actively include Military, Federal, Police, or Sherriff forces, nor Guardsman nor reservist all of which may or may not be part the estimate.

75% of those with Low Proficiency can be quickly trained to reach Moderate skill and 75% of the Poor proficiency can be likewise trained to reach either Low (60%) or Moderate (40%).

That makes for one hell of a free-standing civilian army.