On the User Flow line, you will see the number of events you can save in a month. An event is something that occurs in the course and gets recorded in User Flow. For example, a click, slide start, menu opened, audio paused, etc. To illustrate this, let's assume you have a course that consists of one slide with a button, and all learners click this one button before exiting the course. If 10 learners take your course, there will be at least 3 events recorded per learner: slide start, button click, course exit. The total event count for all 10 learners will be 3x10=30 events.
There is a default email notification that goes out when you exhaust 80% of the credits for the month, so this will give you enough time to make arrangements if needed. You can choose to upgrade to the next plan or do nothing. If you run out of credits, the User Flow events will no longer be recorded for the remainder of the billing period, and the credits will reset with the next billing cycle. You can verify that the 80% notification is turned on (or turn it off if needed) on the Account page.