The barometric pressure in Breckenridge, Colorado (elevation 9600 feet) is 580 mm Hg. How many atmospheres is this?

Well it is certainly less than the pressure at sea level.

One atmosphere at sea level can support a column of mercury ##"760 mm"## high.

We make the the calculation: ##"580 mm Hg"/("760 mm Hg"*atm^-1)##, approx, 3/4 of an atmosphere. I can remember hiking (slowly!) at those altitudes, and it certainly was difficult to get your breath given the reduced concentration of oxygen at these altitudes.