Actually, the number of deaths would be negligible. Yellowstone sits in the most sparsely populated region of the U.S. The actual direct destructive power of the volcano would only effect a 40 square mile area, which except for Jackson Hole, is largely empty.
Laramie, Cheyenne, Bozeman, Billings, etc would be hit hard by ashfall, but Denver would only get about a foot. Folks know ash collapses roofs. So, gasp, folks would clear the ash as it accumulates. Many or most people would evacuate anyway.
This is alarmism. At its worst, there will be an immense disruption of the electrical and telecommunications grid, immense expense from ash damage and removal, alot of immediate deaths and some ash deaths.