403.3. When Recycling is listed as a Cost , the action must be able to be completed for the cost to be paid. Example: Vi, Destructive has the ability "Recycle 1 from your trash: Give me +1 [M] this turn." Each time a player activates the ability, they must recycle 1 card from their trash to pay its cost. If they have no cards in their trash, they can't activate the ability, because they can't pay its cost.
403.6. This action, when instructed, is sometimes formatted as " Recycle X from [Zone]." That means to take X cards of the instructed player's choice from the relevant zone and recycle them. Example: Dr. Mundo, Expert has the ability "At the start of your Beginning Phase, recycle 3 from your trash." As that ability resolves, its controller recycles 3 cards from their trash. If there are fewer than 3 cards in that player's trash, they recycle as many as they can.