CREATE TABLE table_51458 ("Class" TEXT, "Operator" TEXT, "No. Built" FLOAT, "Year Built" TEXT, "Cars per Set" FLOAT, "Unit nos." TEXT)

What class has less than 12 numbers built operated by southern?
SELECT "Class" FROM table_51458 WHERE "No. Built" < '12' AND "Operator" = 'southern'
SELECT "Class" FROM "table_51458" WHERE "No. Built" < '12' AND "Operator" = 'southern'