The price is for 2 square feet (1x2).
This is 1/8" thick high temperature ceramic gasket material. If you plan on using a steel anywhere in your construction, you need this or the thicker cermic blanket material to seal the steel and make an air tight connection. The steel expands and contracts much quicker than flues, stone or mortar. This will cause the steel to break the mortar. If the steel has a layer of ceramic blanket between it and the mortar or stone, the steel can expand and contract without braking the air tight seal.