Whenever Sage of the Falls or another non-Human creature enters the battlefield under your control, you may draw a card. If you do, discard a card.
You draw a card and discard a card all while Sage of the Falls’s ability is resolving. Nothing can happen between the two, and no player may choose to take actions.
If more than one non-Human creature enters the battlefield at the same time, Sage of the Falls’s ability triggers that many times. You’ll resolve the abilities one at a time—you won’t draw that many cards at once and then discard that many cards.