I read the book of animal fact.
It said that they are 4,000 mammals and that the biggest group of mammals have 1750 types in it.
In wonder why the mammals are divided in different groups. I think is because if they are in one whole group and the scientist want to study them it will be harder. They will need to divide them in groups and need to find there scientific name,but the hardest thing will be to distinguish if they are dangerous mammals to humans.