All field tile is purchased by the full box. Field tile is the term for the plain tile used on floors and walls. The square footage in a box of tile varies by manufacturer. The average range is a couple square feet up to about 30 square feet. Showroom staff can tell you how each tile is packed.
Trim pieces like tile bullnose, tile quarter round and decorative moldings are sold by the individual piece in most cases.
Decorative tile is often sold by the piece and typically there is no minimum order for decorative tiles. Usually, you can purchase as few, or as many, tiles as you need.
Mosaics for shower floors and decorative accents are generally sold by the individual sheet. A sheet is typically 12" x 12", but this does vary. There are some exceptions; a few manufacturers require purchase of full boxes of mosaics. In this scenario, there are about 5 sheets or 5 square feet per box.