!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?

No — the +3 is lost, and Elnuk Herd doesn't stay.

Played from Hidden, Elnuk Herd's play effect must choose its target from units at the battlefield it was hidden at (Rule 811.1.d.2), and that target locks in when the trigger finalizes. When you react with Thrill of the Hunt, it resolves first: the unit is banished to Banishment (a non-board zone) and then replayed, so it re-enters as a brand-new game object. That severs the targeting relationship — Elnuk Herd's "give another friendly unit +3 [M] this turn" instruction mistargets and is ignored, so that unit gets no bonus at all, even if you replay it to the same battlefield. (FAQ #9870 confirms the unit loses all modifiers and attached gear when it's banished and replayed.)

Elnuk Herd also doesn't stay: "then recycle me if I was played from face down" is a separate instruction whose only condition is how it was played, not whether the +3 landed, so it still recycles itself into your deck once the trigger resolves.