Whichevereth does not appear to be listed in dictionaries.
With only a few Google hits, across a selection of informal texts and snippets, it is perhaps used to indicate that the speaker does not care about the ordinal position of the object in a long row thereof:
And indicating with a nod a bespectacled and nonthreatening Mancunian solicitor with whom he was apparently vying for the latest Louis the whichevereth bureau, [...]
In this sense, it seems to be a cliché, largely synonymous with umpteenth.
However, whichevereth is perhaps not immediately understandable in this sole sense (i.e., as a pronoun pertaining to order). Indeed some speakers use it as an archaic form of whichever:
And ye shall face the snorkel whichevereth way thy boltholes lineth up or there will be great gnashing of teeth.
whether whichevereth is largely understandable in the sense pertaining to order
whether it can be used in a question (such as, a question about position in an order).