Each creature you control that has an Adventure enters the battlefield with an additional +1/+1 counter on it. (It doesn't need to have gone on the adventure first.)
If an effect refers to a creature card, creature spell, or creature that has an Adventure, it won’t find an instant or sorcery spell on the stack that’s been cast as an Adventure.
A creature with an Adventure you control that would enter the battlefield with no +1/+1 counters on it enters with one +1/+1 counter on it.
An effect may refer to a card, spell, or permanent that “has an Adventure.” This refers to a card, spell, or permanent that has an adventurer card’s set of alternative characteristics, even if they’re not being used and even if that card was never cast as an Adventure.