I have 3
12a has a D designation
12b has a W designation
12c has a DD designation
Also note that once you enter these codes into the W-2 screen, you don't normally need to enter them elsewhere on the return - TurboTax will handle them for you.
It's important to know that the amount coded with W in box 12 is the sum of the employer's contributions to your HSA and also your contributions to your HSA through a salary reduction/payroll deduction plan. This entire amount is called "employer contributions" by the IRS.
This is important because there is a screen in the HSA interview called "Let's enter [name] HSA contributions" (see screenshot). The second line asks for your HSA contributions, so many taxpayers are tempted to put their amount of payroll deductions on the second line. But the payroll deduction is already part of the first line, so you should not enter the code W amount here.
P.S., the D is for your 401(k) plan contribution and the DD is the sum of your employer's and your contributions to your health plan.