How many scenes are in movies?

It depends on the type of movie. Short films can have a couple of scenes and others can have many. The writer and director get together to agree on how the movie will be made and figure out the amount of scenes.

As noted, depends on various factors. As a screenwriter the best answer is "as many as you need". Usually, the rules are more about amount of pages and what each scene must do.

A first time screenwriter should stick with 90-120 pages. Each and every scene should move the plot forward.

In an first script, a new screenwriter should aim to have it all take place in one spot. A good example is "Albino Alligator". You can probably find the shooting script for sale, and/or watch the film carefully.

As for plot development, in an average 120 page screenplay.

First 30 pages: Introduce the main character(s) and end page 30 with a problem the hero/ine seems impossible he/she can get out of...

[on the edge of a cliff--150 feet drop and coming toward her, a man determined to hurt her with a 2-gage shotgun. Jump to her death or get shot and fall off the cliff?]

Pages 31-90 (approx): The action! [She jumps and suddenly the bad guy hears and sees her jet pack start and she "flies" quickly away--the pursuit has begun]!

Pages 91-120 (approx): Climax...At last, it is the final battle, when all storylines come to a conclusion/outcome. The heroine usually destroys the antagonist. Although endings are about choice; it can be ambiguous or maybe tragic or happy [Heroine battles and destroys the killer, saving herself and her family, or a city or the planet, etc]

If it takes 200 scenes or 400, the screenwriters choice; and once in production--the director--post production--the Editor (probably with the Editor). Many scenes ending up "on the cutting room floor".

You can order "shooting scripts" on-line with each scene numbered.