Mormons are, so far as I’m aware, the only Christian religion that has a specific priesthood office of “Seventy.” The charter for this is to be found in KJV Luke 10:1:
After these things the Lord appointed other seventy also, and sent them two and two before his face into every city and place, whither he himself would come.
The Greek text is as follows:
Μετὰ δὲ ταῦτα ἀνέδειξεν ὁ κύριος [καὶ] ἑτέρους ἑβδομήκοντα[-δύο] καὶ ἀπέστειλεν αὐτοὺς ἀνὰ δύο πρὸ προσώπου αὐτοῦ εἰς πᾶσαν πόλιν καὶ τόπον οὗ ἔμελλεν αὐτὸς ἔρχεσθαι
There are two textual issues here. The conjunction kai (which I’ve placed in brackets) is present in the Textus Receptus tradition upon which the KJV is based, but probably does not belong. And the textual evidence is pretty evenly split between the number being 70 or 72.
So the beginning of the verse means something like “And after these things the Lord appointed seventy[-two] others…” If we include the conjunction, it would have an intensive meaning: “the Lord appointed even seventy[-two] others…”
The word “others” (heterous) is a plural to match the plural number 70/72. The word is used to indicate that these 70/72 were sent out in addition to the 12 previously sent out. But the KJV translators for some reason felt the need to follow the Greek word order in their English rendering, even though there is no need to do so. And in Greek the word for others precedes the number 70/72. One cannot very well say “others 70”; if “others” comes before 70, in English it needs to be singular: “other 70.” Since the KJV translators were following the Textus Receptus, they reflect kai in their rendering, using the word “also,” giving us “other seventy also.”
I propose that this apparent singular use of the word “seventy” is what led to our modern LDS practice of using the word seventy as a title for an office, in which an individual (singular) can be “a” Seventy.
The number 70 (and 72 for that matter) is deeply symbolic in the scriptures, so from that perspective I think it’s actually a pretty cool title. But I’m guessing it had its modern genesis in the ultra-literal translation of KJV Luke 10:1.