As long as you want! As short as you want, the end result is an expensive raised bed; cheaper to just to use 4x4s and 1x1s treated wood if you ask me. Though, I must say, when I was building my terrace garden beds, the first thing that popped to mind, was the concrete blocks for stability.
At any rate, you have the dimensions of a standard concrete block: 16-inch-long, 8 inches high and 8 inches wide. You want to build a bed that is 16 feet long and 4 feet wide.
The length of the bed is 16 feet. There are 12 inches to a foot, so you will need to multiply 12 and 16 which equates to 192 inches. How many blocks are required for 192 inches? You will need to divide 192 inches by 16 inches which equates to 12, 12 blocks per side for the length(Long), for a total of 24 blocks.
The width of the bed is 4 feet. There are 12 inches to a foot, so you will need to multiply 12 and 4 which equates to 48 inches. How many blocks are required for 48 inches? You will need to divide 48 inches by 16 inches which equates to 3, 3 blocks per side for the width, for a total of 6 blocks.
For both the width and length, that brings the total amount of blocks to 30 per layer assuming total width and length of the entire bed with the blocks. Remember, your inside measurements for the width will be short by 8 inches on each side for the width (one full block), as you have to account for right angles on the outside of the bed; so as she stated, just add 1 additional block per side for the width to get a working area of 16 feet by 4 feet (Inside); then the total of blocks will be 32. Hope this helps.