CREATE TABLE table_64028 ("Class to 1928" TEXT, "Class from 1928" TEXT, "Seats" TEXT, "Quantity" FLOAT, "Year ( s )  of Manufacture" TEXT, "Remarks" TEXT)

What is the smallest quantity having a Class to 1928 of BDi-21?
SELECT MIN("Quantity") FROM table_64028 WHERE "Class to 1928" = 'bdi-21'
SELECT MIN("Quantity") FROM "table_64028" WHERE "Class to 1928" = 'bdi-21'