CREATE TABLE table_25695027_1 (number_built VARCHAR, valve_gear VARCHAR, cylinder_size VARCHAR, firebox VARCHAR)

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