Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
How many engines were built with a cylinder size of 20 ½” x 26”, firebox is belpaire and valve gear is from Stephenson?

### Inputs:
table_name: table, columns: ['Class', 'Firebox', 'Cylinder size', 'Valves', 'Valve gear', 'Number built', 'Years built']

### Response:
SELECT Number built FROM table WHERE Cylinder size = 20 ½” x 26” AND Firebox = Belpaire AND Valve gear = Stephenson