If you recycle more than one card, does Karma only buff one unit or is it once per card recycled, for instance if Baited Hook recycles 4?

According to the RiftJudge FAQ (specifically FAQ #212 and #3168), Karma triggers only once when multiple cards are recycled in a single instance.

Because Baited Hook instructs you to "recycle the rest" of the cards (those remaining from the top 5 after you've banished one) simultaneously as a single game action, this counts as one instance of recycling. Therefore, Karma will trigger only once and grant only one buff, regardless of whether 1, 2, or 4 cards are recycled.

As noted in the FAQ, for Karma to trigger multiple times, the effect would need to be a reflexive trigger that recycles cards one at a time in a sequence.