Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_28016 ( "Class" text, "Firebox" text, "Cylinder size" text, "Valves" text, "Valve gear" text, "Number built" text, "Years built" text )

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

### Accurate SQL ###

SELECT "Number built" FROM table_28016 WHERE "Cylinder size" = '20 ½” x 26”' AND "Firebox" = 'Belpaire' AND "Valve gear" = 'Stephenson'