There is 2 section for box 12 there's 12a and 12b is this for Credit for Retirement Savings ? if so do i need to add both cause there is only one box for it on this form?

There are 4 boxes 12 on TurboTax's W-2 form, 12a through 12d.  Enter the amount from boxes 12 of your W-2 onto TurboTax's W-2 form.  Enter them nowhere else in TurboTax.  TurboTax will automatically include on Form 8880 the amounts entered on TurboTax's W-2.