The basic number of scenes within a script will vary with genre. Keep in mind that each page of script represents approximately one minute of screen time, with the average scene not to exceed three pages. Keep in mind that most scripts submitted, also genre dependent, come in between 100-110 pages.
The more talky the genre, such as romantic comedies and character dramas, the longer the scenes (three pages plus), and thus overall you’ll have fewer scenes within your script. I’ve heard that these scripts generally average 60-90 scenes (when production numbered), which gives the writer plenty of room to have those key 5 and 6 page scenes, if they need them.
Conversely, an action, action-adventure or thriller script, where people aren’t talking so much as running around, will have many more scenes, because a scene might constitute an eight of a page or less. These scripts can have 90-120 scenes or more, depending. I’ve written a couple of thrillers and when production numbered, they seem to come in around 160 scenes.
When you’re beginning the outlining process, if you write your scenes on index cards, I generally find that 10-12 index cards per act section (act 1, first half of act 2, second half of act 2, act 3) is generally sufficient material. If you write a treatment based on the cards and hit 10-15 pages, you’ll have enough material to get through a first draft. Then comes the fun part: rewriting.