How many blocks to build a retaining wall 21ft long and 3ft high?

Depends on the size of the block.

However, an house brick is 9" x 3" x 4"

So assuming the brick is laid 9" long and 3" high a single course of wall would require!!!

21 x 12 = 252 "

Hence 252 / 9 = 28 bricks for the length

&

9 x 12 = 108 "

Hence

108 / 3 = 36 bricks in height

Hence 36 x 28 = 1008 bricks is required!!!!

If you wanted a double course wall (for support) that is 8 " wide. Then you need twice the number of bricks which is 2016 bricks.