The lower the number of teeth on a rear cog, the more development inches you create (the distance you can move your bike with each rotation of the crank) but it yields higher top speeds pedaling at the same RPM.

Say your chainring is a 48 tooth and you have an 18 tooth rear cog (and youre running a typical 700x23 wheel/tire combo). One full rotation of the crank will move your bike 220 development inches and at 100rpm, you'd be going 20.8 mph.

If you stayed with that same 48 tooth chainring but dropped down to a 16 tooth rear cog, you would have 248 inches of movement with the same single rotation of the crank/pedals and at 100rpm, your speed would be 23.5 mph.

A good way to decide the number of teeth you WANT on a rear cog, depends on what size your chainring is. It also matters if youre in a predominantly hilly area or flat area or if you ride fast or casually slower. You create a "Gear Ratio" when you pair a chainring with a cog and that ratio is extremely particular to the individual. I run a 49x16 in a mildy hilly but predominantly flat area of Baltimore City with no brakes. Sucks on the hills but great for riding across the harbor area of town where its flat