A complete end-to-end flow of a journey is regarded as one transaction. Each time a journey is triggered, it is considered as one transaction even if a journey fails, succeeds, or remains in a pending state—no matter how many steps you have in any given journey.
For example, you have a journey that fetches a message from Facebook Messenger and forwards it as an email to a Gmail account. Right from listening to messages on Facebook Messenger to sending each of them as an individual email, the complete journey execution counts as one transaction.
The number of transactions performed during a journey execution can be found under the History tab. The number of transactions you have available depends on the subscription you have purchased.
How to count a number of transactions of a journey
Whenever a trigger in a journey is executed, it is counted as one transaction. By default, a trigger in a journey runs only once and is always counted as one transaction even if it fails to return an event.