My veggie beds are only 8". They work just fine for me.
HTH

It would depend on the need for the raised bed. I must have a raised bed due to potential flooding, so mine is around 8" and it works for almost all circumstances.

Gary

I have been using raised beds because I find it is easier to control weeds. In general the square foot gardening method makes sense to me and I have been able to grow almost anything that will grow in zone 10 successfully (one exception being zucchini, just no luck with them).

Anyways, 6-8" should be sufficient. I usually put something underneath the soil, cardboard or several layers of the weed-barrier fabric commonly available at Home Depot or Lowes. Then I have the liberty of formulating the soil mix any way I want it.