You choose how the counters will be distributed as you put the reflexive triggered ability onto the stack. Each target creature must be assigned at least one counter.

You may choose zero target creatures for Yannik's reflexive triggered ability. If you do, no creature receives +1/+1 counters.

If Yannik leaves the battlefield before its triggered ability resolves, you won't exile any creature you control. Yannik's reflexive triggered ability won't trigger.

Auras attached to the exiled creature will be put into their owners' graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist. When the card returns to the battlefield, it will be a new object with no connection to the card that was exiled.

If a token is exiled this way, it will cease to exist and won't return to the battlefield. It will still let the ability distribute counters.

Yannik's triggered ability doesn't target the creature you control. That creature is chosen as the triggered ability resolves. After it's exiled, the reflexive triggered ability triggers and targets are chosen to receive the counters.

Use the power of the creature as it last existed on the battlefield to determine how many +1/+1 counters to distribute.

If one of the target creatures becomes an illegal target in response to the reflexive triggered ability, the +1/+1 counters that would have been put on that creature are lost. They can't be put on another legal target.

Yannik can be the target of its own reflexive triggered ability.