Your question doesn't exactly make sense.
Ampacity calculations are based on the insulation type of the conductor along with adjustment and correction factors that may be present.
Assuming neither of those issues are present, a 20A branch circuit supplying receptacles must utilized conductors no smaller than #12 AWG. This is specified in 210.19 and 240.4 of the NEC.
The type of insulation used on your #12 AWG conductors will determine your ampacity, that being 90, 75, or 60 degree. According to the Table, a #12 AWG conductor can be loaded to 25A before its temperature reaches 60 and 75 degrees. So in this case, any insulation type can be used for your branch circuit.
The 20A branch circuit rating protects the conductor, termination ratings, and device ratings.
Now, assuming an adjustment or correction is needed, the code permits you to utilize the highest rating of the conductor insulation to perform the derating calculation, but when all is said and done, the equipment and materials cannot be loaded greater than the minimum rated component of the system.
