Let's crank this bad boy up to a full blown migraine.
We have the 4gauge, 8gauge, 10gauge, 12 gauge, 16 gauge, 20gauge, 20gauge, 32gauge, and .410. The 410 is actually a 67gauge using the "gauge" terminology. I've also seen listings for a 14gauge and 24gauge, but have never seen loaded ammo for it (nor for the 32 for that matter).
Keep in mind, some of the larger gauges were only around during the blackpowder days. As powder and guns became more efficient, the need for such large shotguns was diminished. These days, a 12 gauge can do the work of an older 10gauge loading and a modern 20 can do the work of the original 12gauge. 8Gauge and larger were outlawed by the Feds a long time ago, but nobody really noticed since they were too large to be practical and smaller gauges had closed the performance gap.
IMO, a 12 is all that's necessary. With the right loads, you can do it all with a 12, but it's nice to have a 20, 28, 410, 10, etc. Variety is the spice of life.
Chris