Just being at altitude at CO for a week will boost red blood cell count. Live high, train low is the mantra actually. Training at altitude has some benefits but just being there is the actual benefit. BTW those altitudes some poster listed above are accurate but most ski areas in CO are significantly higher than that
Summit County at the low spot is 8200 ft but most areas there top out at or above 12,000 ft. (Breck, Copper, A Basin actually get close to 13,000) ...if you really want a workout rent some AT ski gear with skins and hike up the ski areas before they open them ski down. You will never experience a harder workout in your life....this is also referred to "SkiMo" short for Ski Mountaineering and is an Olympic sport starting in 2026.
Have fun, just ski or maybe light run a little. Red blood cell boost will come anyways if you're there for a week.