This guide suggests 6-10 inches: https://homeguides.sfgate.com/deep-till-vegetable-garden-67694.html
Here are some helpful videos for you on how deep to go for your garden!
10” is a lot of physical work and I’d suggest closer to the 6” range.
Hi Edna: Six inches is good. You can till in some soil amendments while you're at it:) Good luck