Most starfish have five arms, but some species can have up to nine. The number of arms a starfish has is usually proportional to the size of its body. For example, the common sunflower star can grow up to two feet in diameter and has as many as 24 arms. On the other hand, the something small pygmy sea star only grows to be about an inch in diameter and typically only has five arms.
The vast majority of starfish are found in tropical or temperate waters, with only a handful of species inhabiting colder climates such as the Arctic Circle. Starfish are generally found on coral reefs, rocks, or the ocean floor where they use their ambulacral grooves (the furrows between their tube feet) to move around. Some species are even able to crawl across wet sand without sinking thanks to these same tube feet.
Starfish are predators that typically eat mollusks like clams and oysters. To do this, they first locate their prey using sensors located on the tips of their arms. They then pry open the shell with their strong suction-like tube feet and extend their stomachs inside to digest the animal within.
Interestingly enough, some larger species of starfish are also known to eat smaller fish, echinoderms (including other starfish), sponges, and even sharks!