Correct spelling, explanation: 12th is the correct form of this ordinal number because the full-word form is twelfth so th sound is at the end. This is the case with ordinal numbers – the last two letters of the full form are the letters we write next to the numeral form. For example second – 2nd, eleventh – 11th, fifty-third – 53rd, etc. This is why 12th is the correct form, not 12nd.

Definition of 12th:
numeral, the ordinal number between 11th and 13th
For her daughter’s 12th birthday Meena prepared a delicious cake.
I was 12th in the line at first, but a few people resigned in front of me and I came into the doctor’s office in less than 20 minutes.

Incorrect spelling, explanation: 12nd is incorrect because nd is not the sound that ends the full form of this ordinal number, but th is (as in twelfth = 12th). This number can be tricky, because of 2 being there at the end (2 is second in its ordinal form, so it ends with nd, hence 2nd). However what matters is how the number is spelled fully, so in the case of twelfth there is th. Thus 12th is the correct form.

Incorrect spelling

Incorrect spelling