Where does the series number on a map appears?
It depends on the scale of the map. Topographic map series offer a consistent scale across the series, usually an even number such as 1:24,000, 1:50,000, 1:100,000, etc. Different countries offer different topographic series scales. For reference, 1:63360 is equivalent to one inch to the mile, so at that scale, a square inch is equal to a square mile.