box 12 has 12a, 12b, 12c, 12d which amount do i use when there's only one box to fill it but my w-2 has amounts in all boxes with capital letters in each

When you are entering your W-2 in TurboTax, the place where you enter Box 12 with the code letter and amount will expand and allow more entries than are initially visible on the screen.  

If it does not expand automatically, click the blue box that says Add another Box 12 item to open another line.