It's asking for Box 12 amount, but there are 4 different Box 12 amounts. 12a-12d, which one is it asking for?

[saferin-1]

What box 12 do I put in for HSA contributions?

[AnnetteB6]

You should enter any and all box 12 information that is reported on your W-2.  It does not matter how many codes and amounts your W-2 is showing for box 12 because the W-2 entry in TurboTax will allow you to enter as many as needed, even if there is more than 4.  

The designations for box 12a, 12b, 12c, and 12d are meaningless because the letter codes beside the amounts is the important information designating what the amount is showing.  

An amount in box 12 of your W-2 with code W beside it represents HSA contributions made through your payroll at work. 

To learn more about the box 12 codes and what they mean, take a look at the following TurboTax help article:

What do the letter codes in box 12 of my W-2 mean?
 

[dmertz]

Amounts shown in box 12 of your W-2 are to be entered only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax.