The deeper, the better.
But you could start with 6-8 inches.
I would not start a bed with less then 12 inches.
I make beds different then most people I know.
I start with a lot say 14 X 40 feet.
I spread compost & lots of browns on the bed, then I till it in.
I mark off 2',4',2',4',2',then I shovel the soil out of the two feet& put it on the 4 foot bed, then the next 2' on the next 4' bed & so on.
I now have two double dug beds, then I use boards, rocks,blocks to hold the soil in place & rake it out smooth as possible. Then every fall I till or turn in browns, compost & coffee waste + lime with a shovel.
I try to put in a new bed or two every year. I have flowers in one of my garden beds this year.
At this rate I may never catch up.

Never go deeper than actually needed 'cause nothing really happen down there(subsoil level). I think the top 15cm would suffice since no deep root crops will be planted.

What you need to do is create a planting environment in which no, or minimal, textural interface occurs. You don't what your backfill soil to be significantly different from the surrounding soil in regards to texture (sand, silt and clay percentages). An abrupt change in soil texture can create drainage problems. This goes for backfilling a hole with sand when the surrounding soil is clay and vice versa. If you're pulling out soil that makes an unfavorable growing medium, be sure to remove it all unless it's a good textural match for your native soil. Then, backfill with a soil that is a good textural match for your native soil. For example, if you have an 18 inch deep hole filled with clay/debris but your native soil is sandy, you wouldn't want to pull out 1 foot of the clay/debris and throw sandy soil on top of the 6 inches of clay/debris at the bottom because you would then have two textural interfaces working against your drainage (interface #1- sand to clay/debris then #2- from the clay/debris back to sand).

If you're going to use a "recipe" that includes organic material and you excavate more deeply than 12 inches, use straight soil for the bottom and place soil blended with organic no more deeply than 12 inches. Not much oxygen gets lower than 12 inches in the soil profile and organic material below that depth will decompose anaerobically (without oxygen), which comes with its own set of problems. If you're looking to use a soil/organic blend, something that is about 80 to 75% dirt and 20 to 25% organic material by volume will typically get you in the neighborhood 5 to 6% o.m. by dry weight. Of course it will vary a little depending on the particular organic amendment chosen. Be sure that whatever you use for organic material is nitrogen stable to avoid a nitrogen draw in the soil as it decomposes. Also, at that high of a rate, consider avoiding a straight manure product due to the relatively high salt content.