Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the smallest quantity having a Class to 1928 of BDi-21?

### Inputs:
table_name: table, columns: ['Class to 1928', 'Class from 1928', 'Seats', 'Quantity', 'Year(s) of Manufacture', 'Remarks']

### Response:
SELECT MIN Quantity FROM table WHERE Class to 1928 = bdi-21