Although a flash with a guide number of 48 has 9 times the light output as a flash with a guide number of 16, for a given f-stop it can illuminate a distance only 3 times further. You can use a flash with coverage wider than the lens you are using with no change to the guide number. The guide number for a given flash unit is different for each film speed.
Guide numbers differ according to whether the distance is to be measured in feet or in meters.
There are several common formulas for flash exposure that involve guide numbers.