So what’s a sequence, exactly? Definition: a self-contained portion of the entire story, usually about 10 to 15 minutes (pages) in length.
Every sequence has its own tension, not necessarily the main tension, but is related in some way. It usually “belongs” to one particular character, not always the protagonist, and aspects of the overall story are kept alive, even in sequences where the overall tension is about something else entirely.
Here’s the gist: we get additional information that advances the story while, at the same time, we have a new short-term tension to worry about. Traditionally, there are eight sequences in most genre features: two in Act I, four in Act II, and a final two in Act III.
Occasionally, the first act may have three sequences, which is quite common within the action genre, and some scripts may include five sequences in the second act. But eight is a good, safe standard number to follow.