!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?

Short answer: the +3 doesn't travel with the unit, and Elnuk Herd doesn't stay either.

Elnuk Herd's Hidden play effect can only choose its buff target from units at the battlefield it was hidden at (Rule 811.1.d.2 — same as the Blastcone Fae example), so no, you can't hide it at A and buff a unit elsewhere. If you then Reaction with Thrill of the Hunt pointing at that same unit, Thrill banishes it first — Banishment is a non-board zone, so the replayed unit is a brand-new game object (Rule 124) and Elnuk Herd's play ability mistargets when it resolves: instructions tied to that unit are ignored, so it gets +0, not +3. Even if the buff had somehow already landed, a zone change strips temporary modifications, so the +3 would be gone anyway (124.1).

Elnuk Herd itself also doesn't stay: because it was played from face down, its own "then recycle me" puts it on the bottom of your deck. That recycle isn't contingent on the +3 landing, so it happens either way.

If you meant pointing Thrill of the Hunt at Elnuk Herd itself instead, the +3 on the unit at that battlefield does still land (its target never became illegal), and the replayed Elnuk Herd is a fresh play so its play ability triggers again for another +3 — but the original trigger still recycles it.