if wind weren't a factor, i could probably be happy with just one (chicago is dead flat), but riding DIRECTLY along the shore of lake michigan as i do everyday, wind is unfortunately a HUGE factor for me, and my absolute minimum of 5 gears would be along the lines of:

~50 - stupid strong headwind
~65 - headwind
~80 - neutral
~95 - tailwind
~110 - stupid strong tailwind

as it is, i ride a road bike with a 2x10 ultegra drivetrain (53/39 x 11-28) and a hybrid with an alfine 8 IGH.