SHY: “But don’t claim “The Rebbe made it up” because it was used before the Rebbe was born.
See Tanya Perek Bais:
וכן בכל דור ודור יש ראשי אלפי ישראל, שנשמותיהם הם בחינת ראש ומוח לגבי נשמות ההמון ועמי הארץ, וכן נפשות לגבי נפשות, כי כל נפש כלולה מנפש רוח ונשמה.
(Note it doesn’t use the words Nosi HaDor, just the concept)”
Nope. The concept that your rebbe made up is clearly in the singular and he only every used it about his father-in-law, and as understood by lubavichers by implication about himself. One Nossi hador and that’s it.
Not only does the Tanya not use that loshon, but he clearly says it in the plural: roshei, nishmoseihem, heim.
Btw, I have heard this Tanya quoted, and explained by a lubavicher that the Baal Hatanya was really referring to himself, in the singular, but he wrote it in plural out of anivus! I kid you not.