How many scenes are in movies?

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.

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.