I own and use G502. It's not going to get you to the 12 individual buttons you're asking for. At most, with the G-shift, you'll get 10. You technically can get more but you're gonna lose your wheel, left and right buttons just to go over.
The G502 has three on the thumb, two to the left click. So not accounting the wheel and basic left-right, you've got 5. Then G-shift will allow it to go to 10.
The G-shift will use a dedicated button, preferably below the toggle speed-wheel button.