Some Replacement Effects will begin with “once each turn.” — Riftbound Rules

Section 371 — 12 rules

371.1.: When an event the Replacement Effect could apply to occurs, the player who controls the Replacement Effect may choose to apply it to the event.
371.2.: If they do not, it has not been applied this turn.
372.: If more than one Replacement Effect applies to the same event being executed, then the owner of the object being acted on determines the order the Replacement Effects will apply.
372.1.: If it is a player being acted on, that player decides the order the Replacement Effects will apply.
372.2.: If the affected object is an Uncontrolled Battlefield then the Current Turn Player decides the order the Replacement Effects will apply.
373.: If more than one event occurs simultaneously that Replacement Effects could apply to, each event is treated separately and individually for the purposes of Replacement Effects, and Replacement Effects with the same controller are applied in the order of their controller’s choosing. Example: Two units controlled by the same player die in the same cleanup. That player also controls Zhonya’s Hourglass. They must decide which event to apply Zhonya’s Hourglass to first.
373.1.: Although these events are simultaneous, the applied Replacement Effects are ordered. If multiple applied Replacement Effects with different controllers would execute simultaneously, they execute in turn order.
373.2.: When applying Replacement Effects to events that occur simultaneously, each Replacement Effect may only be applied in one sequence, to any number of events that are qualified to be replaced. Example: Soraka, Wanderer reads “If another unit you control here would die, if it has less Might than me, instead heal it, exhaust it, and recall it.” Soraka dies simultaneously with two 1 [M] Recruit tokens at the same battlefield and two 1 [M] Recruit tokens in base. Soraka has a Guardian Angel attached to her when she dies, which appends “If I would die, kill Guardian Angel instead. Heal me, exhaust me, and recall me” to Soraka’s rules text. There are several possible ways to order the Replacement Effects being applied to the various events: If Soraka’s Replacement Effect is applied first, it saves the Recruits at the same battlefield as her but not the Recruits in base. If the Replacement Effect appended by Guardian Angel then saves Soraka, she cannot apply her Replacement Effect to the Recruits in base as her Replacement Effect has already been applied to an event simultaneous with it dying. If the Replacement Effect appended by Guardian Angel is applied first, it saves Soraka and recalls her - then when Soraka’s Replacement Effect is applied, it can only save the Recruits in base.
373.2.a.: A sequence of Replacement Effects is an uninterrupted series of applications to a set of simultaneous events.
373.2.a.1.: A Replacement Effect that replaces an event or Game Action that is part of another Replacement Effect will not interrupt the sequence of the replaced Replacement Effect’s application.
374.: A Replacement Effect’s controller is the player that controls the source of the Replacement Effect.
375.: If an event that a Replacement Effect applies to would be modified by a Game Effect, or the results of that event would be modified by a Game Action, the Replacement Effect will inherit those modifications. Example: Treasure Hunter reads “When I move, play a Gold gear token exhausted.” A Replacement Effect that says “if you would play a token gear, play that token and an additional copy instead” is applied to the event of the Gold gear token being played. The additional copy will also be exhausted, as it inherits the “exhausted” modification. Example: Another Replacement Effect says “if you would play a token, draw 1 instead.” The modification cannot apply, so we ignore it. Example: A spell reads “play a ready 3 [M] Mech token. Then do this: Give it Temporary.” A Replacement Effect that says “if you would play a unit token, play that token and a 1 [M] Recruit token instead” is applied to the event of the Mech token being made. The Recruit token enters ready and is given Temporary.