Correct spelling, explanation: twelfth is the correct form because v in twelve becomes f after adding the suffix th. This is due to the fact that both th and f are voiceless sounds, so they stick together. We can’t however skip this sound at all so twelth is incorrect and the correct form is twelfth.

Definition of twelfth:
numeral, the ordinal number between the eleventh and thirteenth
This is my twelfth time in Rome, I am just obsessed with this city.

Incorrect spelling, explanation: this is an incorrect spelling since it has f (v) sound skipped, which shouldn’t be done. There is v sound in twelve, therefore, it also exists in ordinal form twelfth however in a voiceless version. This is why twelth is a wrong spelling and the correct one is twelfth.

Incorrect spelling