You filled the raised bed right to the top in April.
Soil delivered by the truckload or poured from a bag is full of air.
There is no single figure, because it depends so much on the mix, but some patterns are reliable.
Some sinking is harmless, and even healthy.
The most common mistake is buying exactly the calculated volume of the bed.
Topping up is a normal yearly job, not a sign of failure.
Sometimes a bed needs more attention than an extra layer on top.
Step-by-step advice, charts and free garden calculators at Garden Voila.