Minecraft wiki entry for Brick_pyramid, associated text: Brick pyramids are even rarer than the current rarest structure, woodland mansions, which were added much later in development.
However, unlike woodland mansions, brick pyramids generate in the same locations in every world regardless of seed, making them easier to locate.
A full Brick Pyramid would contain 43,680 brick blocks. That is equivalent to 682.5 stacks of brick blocks.