it's clear there's an amazing diversity of cacti and succulents. Though altogether, there aren't that many different species.
Green Nature says:
They are the largest family of succulent plants, with most of the 2000 known species native to North, Central and South America. The vast majority of cacti, but not all, are well adapted to desert conditions, using their stems to store water during extended dry seasons.
Cacti are flowering plants that serve an important role in their ecosystem by providing food and shelter to many animals, birds and reptiles. Desert tortoises, for example, often snack on their local cactus stems and fruits.