Incorrect spelling, explanation: this form is mistaken, and the correct one is twelfth, because v is a voiced consonant, so we cannot put it in front of the t, which is unvoiced. Instead, we use another sound, f, which is the same sound as v, but unvoiced. Therefore twelvth is incorrect, and the correct form of this ordinal number is twelfth.

Correct spelling, explanation: this form, twelfth, is the correct ordinal form of the number twelve. Although there is a v in cardinal form, it is changed to f in ordinal form, because the sound becomes unvoiced. This happens due to the fact, that t, which is in the suffix, is also unvoiced. As v and f are a sound, but only f is unvoiced we change it. This means that twelvth is an incorrect form and the correct one is twelfth.

Definition of twelfth:
numeral, the ordinal number between the eleventh and thirteenth
They had 11 children and Sarah is their twelfth one.
This is the twelfth time I’m watching this film. I just love it!
Jason proposes to Clara in the twelfth episode of the series.