Lots. With mere mortals unlikely to tolerate low cadences better than professional riders we need gears half the size at 50% less power.

This ignores the fact that pros are too big to compete at 7% body fat, while 74% of American adult men are over-weight and 36% obese.

A 5'9" guy like Alberto Contador who races at 137 pounds is considered overweight at 169, and obese at 203.

Riding a 15 pound bike at the UCI minimum weight with 4 pounds of water/shoes/gear makes combined weights of 156 pounds when racing-fit, 188 over-weight, and 222 pounds obese; calling for 21% and 42% increases in gearing to maintain the same cadence.

Climbing a moderate 8% mountain like l'Alpe d'Huez where pros use 39x23, at half the power 5'9" guys who look like POW survivors should be riding gears like 30x36, the barely over-weight 26x36, and obese 22x36.

Those of us with 10% grades should use 20% smaller granny rings when going all-out.

Obviously things like all day endurance rides in the mountains and loaded touring call for even lower gears.