You can make it as deep as you want.
I'm building my house now with a fully inground basement. I had the basment company pour solid 10' high concrete walls.
The height from the concrete floor to the bottom of the I-Joists is 9' 8" due to the 4" floor.
I highly recommend adding egress window openings if fully inground even if unfinished. You don't want to go back after and install them. I also highly recommend an exterior staircase to get large things in and out if fully inground.
The first floor is 3,337 sq ft so the basement is probably 3,100 sq ft. We were going to do 9' but the extra 1' height added very little to the cost.
If you have the attic space, I'd put the HVAC up there and keep the basement ceiling free of duct work.