In Final Fantasy VII, there are four pieces of Huge Materia.
The Huge Materia quest takes place during the second half of the game, when Shinra tries to seize the enormous crystals for its own purposes. Cloud and his companions pursue four separate pieces, each associated with a color: blue, green, red, and yellow. They are recovered through missions involving North Corel, Fort Condor, the sunken submarine, and Rocket Town.
The number can be confusing because the game allows players to fail some of these missions. A failed objective may leave the party with fewer than four pieces, but the total number placed in the story is still four. Collecting all four also unlocks access to Bahamut ZERO when the required Bahamut Materia are present.
Huge Materia should not be confused with ordinary Materia, the smaller magical orbs equipped to weapons and armor. The four large crystals are mainly story items, while regular Materia provide spells, summons, commands, and passive abilities.