CREATE TABLE table_25695027_1 (number_built VARCHAR(255), valve_gear VARCHAR(255), cylinder_size VARCHAR(255), firebox VARCHAR(255))
How many engines were built with a cylinder size of 20 ½” x 26”, firebox is belpaire and valve gear is from Stephenson?
    
SQL: SELECT number_built FROM table_25695027_1 WHERE cylinder_size = "20 ½” x 26”" AND firebox = "Belpaire" AND valve_gear = "Stephenson"