!ask how does elnuk herd from hidden target a friendly unit on that battlefield, then react thrill of the hunt on elnuk herd, retargeting that friendly unit work? does elnuk herd stay and how many might does that friendly unit get?

Yes — Thrill of the Hunt is legal here and the friendly unit can get buffed twice, and Elnuk Herd stays on the board.

Hidden only locks the first trigger's choice: because Elnuk Herd was played from facedown, its "another friendly unit" must be chosen from units at the battlefield where it was hidden (Rule 811.1.d.2), and it can't be Elnuk Herd itself. That target is locked when the trigger is finalized, so Thrill moving Elnuk Herd doesn't let you re-choose it — as long as that unit is still there, it still gets its +3.

Thrill banishes Elnuk Herd and its owner replays it to any battlefield; it re-enters exhausted as a brand-new object, so "when you play me" fires a second time. That second trigger wasn't played from facedown, so it has no Hidden targeting restriction and no recycle clause — it can pick that same friendly unit (the card has no "here"), and "this turn" Might modifiers stack, so the unit ends up at +6 total.

The recycle in the first trigger ("recycle me if I was played from face down") refers to the Elnuk Herd that triggered it, which is a new object after the banish — the self-referencing instruction can't find it and is ignored (Rule 359.3.e.6, referent rule 359.3.f; cf. FAQ #3430 on "me"/"here"). So the replayed Elnuk Herd stays.

No official FAQ covers this exact line — the "recycle me" part is the referent rule applied to the printed text.