All of them, only one at a time and I usually need one more lower than my lowest.
Never had I the occasion to wish for a bigger, faster gear but, woe is me, I have wished for a smaller, slower, easy to climb with gear.
When I was a kid in Missouri, my bike only had one gear.
When I went back there a couple of summers ago and rode my old paper route, I realized how many fewer hills I would have walked up 40 years ago if I had had gears.
One of the first things I did to my road bike was to switch out the front chain rings for lower geared mountain bike rings.