Correct spelling, explanation: because th sound is voiceless it makes sounds standing before it voiceless as well. This way v in twelve transformed to f in ordinal number twelfth, because v and f are the same sound, the only difference is that the latter is voiceless. Therefore twelfth is the correct form and twelveth is incorrect.

Definition of twelfth:
numeral, the ordinal number between the eleventh and thirteenth
The twelfth contestant in this Miss Eingeneer pageant was my favourite.

Incorrect spelling, explanation: ordinal number from cardinal number twelve is twelfth because v became voiceless after adding another voiceless sound th to it. Languages usually tend to be lazy as their users make the speech easier and easier as they speak. This is why twelveth is an incorrect form, and the correct one is twelfth.

Incorrect spelling