Why are there many kinds of vitamin B, while other vitamins are not?

First of all, vitamin typing is not only group B, but also more than one form of vitamin A/D, such as A 1 and A2, but their structures are similar. VA, VD, VE and VK are fat-soluble vitamins, while vitamin B and vitamin C are water-soluble vitamins. Among them, B vitamins are a large class of molecules that affect substance metabolism by forming coenzyme in organisms. Many B vitamins, such as pantothenic acid, biotin and folic acid, have no labels (namely vitamin BX) but also belong to B vitamins. Vitamin C, also called ascorbic acid, mainly plays the role of anti-oxidation and anti-aging. As for the serial number problem you mentioned, it depends on the order of discovery, but there is no need to delve into it too much.