user: Question: How many engines were built with a cylinder size of 20 ½” x 26”, firebox is Belpaire and valve gear is from Stephenson?
Columns: ['Class', 'Firebox', 'Cylinder size', 'Valves', 'Valve gear', 'Number built', 'Years built']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['E2', 'Radial-stay', '20 ½” x 26”', 'Slide', 'Stephenson', '88', '1901-02']
SQL:

assistant: SELECT SUM("Number built") FROM "Table" WHERE "Cylinder size" = '20 ½” x 26”' AND "Firebox" = 'Belpaire' AND "Valve gear" = 'Stephenson';