You choose whether to put +1/+1 counters on the creature or create a Spirit token as the ability that includes the endure instruction is resolving. No player may take actions between the time you choose and the time that counters are added or tokens are created.
The value of X is determined as the last ability resolves. If Warden of the Grove is no longer on the battlefield at that time, the number of counters on it as it last existed on the battlefield is used to determine how many +1/+1 counters to put on the creature that endured or the power and toughness of the Spirit token created.
If you can’t put +1/+1 counters on the creature for any reason as an endure ability resolves (for example, if the creature is no longer on the battlefield), you’ll just create a Spirit token.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Alexander Ostrowski
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
If a noncreature permanent is instructed to endure, the effect is the same. You can put +1/+1 counters on that permanent or create a Spirit token.
