Kinscaer Sentry
First strike, lifelink Whenever this creature attacks, you may put a creature card with mana value X or less from your hand onto the battlefield tapped and attacking, where X is the number of attacking creatures you control.
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Official rulings (4) · Scryfall
Although the creatures put onto the battlefield with Kinscaer Sentry enter as attacking creatures, they were never declared as attacking creatures. Abilities that trigger whenever a creature attacks won't trigger when the creatures enter attacking.
You choose the player, planeswalker, or battle the creature you put onto the battlefield is attacking. It doesn't have to attack the same player, planeswalker, or battle as Kinscaer Sentry.
The value of X is calculated only once, as Kinscaer Sentry's last ability resolves.
If a card in your hand has {X} in its mana cost, X is 0 for the purpose of determining its mana value.