Music for a picture is not written nor intended to be performed like a symphony... as one continuous event (from the title through the end credits). A film score is actually a set of "chunks" of music. And the word 'cue' is used to denote one of these chunks to be written and performed in relation to one of the scenes or segments in the picture. This piece of music or 'cue' (sometimes also called a 'start') is usually separately titled as well as given an identifying number or code for organizational and administrative purposes. Composers usually make up the titles for each of their cues. But its not unusual for that job to be left to the music editor. Traditionally, the code or number given to a cue is designed to reference that cue to the scene in the film to which it relates. So the most common numbering scheme for film 'cues' has been to identify each by the reel number and then the numerical position or sequence the cue has in relation to other cues on that reel. For example, the second piece of music or cue in reel six would be coded '6M2' (the 'M' just means 'music'). The same scheme usually applies in TV production (i.e., reel#+"M"+music sequence#) since almost all such production is still shot to film.
The number of cues in a picture and the length of each cue can and do vary widely, of course, depending on the nature of the picture and the work habits and artistic inclinations of the composer, the music editor, the director, etc. Here's a couple of pragmatic observations about 'cues': (1) Longer cues are more difficult to 'get right' in the performance - just more of an opportunity for an airplane to fly over, a pencil to drop, or for a conductor or musician to 'clam'. So most composers keep their cue lengths manageable (cues over five minutes are infrequent). If the film requires an exceptionally long, continuous run of music at some point, the composer will usually split it up into two or three 'cues', each to be separately recorded then spliced together later by the music editor. (2) As budgetiers well know, the total number of minutes of music required for a picture is not always indicative of the amount of recording time required. The total number of 'cues' or 'starts' must also be considered. A film that has 60 cues totaling one hour of music will usually take longer to record than a film with 40 cues and one hour of music. This is because there is always a certain amount of technical and music support 'overhead' involved on the scoring stage between 'takes'. More cues = more takes = more overhead = more time = more $$$ to record the score.