Until end of turn, creatures you control have base power and toughness X/X, where X is the number of cards you own in exile and in your graveyard that are instant cards, are sorcery cards, and/or have an Adventure.
The set of creatures that Candlekeep Inspiration applies to is locked in as it resolves. Creatures that enter the battlefield under your control after it resolves will not be affected.
The value of X is locked in at the time that Candlekeep Inspiration resolves. The power and toughness of creatures affected by it will not change as cards enter and leave exile and/or your graveyard.