Incorrect spelling, explanation: since v and f are voiced and unvoiced versions of the same consonant they sometimes change places, depending on the letters before and after them. In ordinal numbers, such as twelfth, the th sound (/θ/), which is voiceless itself, conditions the sound before it to be voiceless as well. Therefore twelve plus th is not twelveth but twelfth – v became a voiceless version of its sound.

Correct spelling, explanation: the /θ/ sound (th) being voiceless, conditions the sound before it to change a little bit, to become voiceless as well. Therefore the /v/ sound at the end of twelve transforms to an /f/, which is linguistically the exact same sound, but voiceless. This is probably due to a very simple thing – usually, language is evolving because of the laziness of its users. English spelling doesn’t always reflect phonetics, however in this case we can see how pronunciation changed the orthography. V transformed to f and therefore twelfth is the correct spelling, not twelveth.

Definition of twelfth:
numeral, the ordinal number between eleventh and thirteenth
I can’t get to the top ten in this game! I’m still twelfth!

Incorrect spelling