Like many others have said earlier, it’s whatever is comfortable for you. Who cares what others ride. It also depends on the terrain and types of roads you ride.
That said, the lowest possible gearing a single speed bike can run is a 1:1 ratio, though you might have a hard time finding a cog and chainring that have the same number of teeth.
I run 70.5 GI on my SS daily commuter. I started with 65GI until my legs got stronger to comfortably handle a higher ratio. My winter SS runs at 64GI to accommodate for snow drag. I live in NYC which is 85% flat/flat-ish and has bridges. But hey, you do you.
Last edited by sdimattia; 02-27-21 at 10:08 PM.