Table schema:
CREATE TABLE table_25695027_1 (number_built VARCHAR, valve_gear VARCHAR, cylinder_size VARCHAR, firebox VARCHAR)

Question: 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"