This is always a difficult question because everyone's situation is different.
10% of Growing Medium
A good rule of thumb, however, is to buy 10% of the volume of your growing medium.
First, you will need to calculate the volume of your growing medium which you can do by taking multiplying your length, width, and depth of your growing area.
So a 4x8-ft raised bed with 1 foot of depth would have a volume of 32 cubic feet (8x4x1). 10% of this is 3.2 cubic feet. We assume roughly 33 lbs per cubic foot.
This is about 100 lbs.
These worm castings would then need to be worked into your soil.
1-Inch Top Dressing
If you do not want to disturb the root zone by working in worm castings to your soil, then you can top dress with a smaller amount as 1-inch of worm castings could be very helpful.
The calculations are the same, except that the depth would be 1 inch...or 1/12th of a foot.....or .083 feet.
8x4x.083 equals 2.6 cubic feet. This equates to nearly 90 lbs of worm castings to top dress a 4x8 raised bed.