This is actually a great question. I'm sure it might bother some purists who believe art should be unfettered. And in many case, I agree, art should have no rules.
But commercial art...
Selling your art in today's marketplace means you gotta follow some rules. If you're looking to sell your completed work, whether as a script or a movie that's made from your script, you have to know that modern audiences are accustomed to a certain rhythm. And if you figure out that rhythm, you can make decent money as a writer.
The first thing I wanna point out is that the word "scene" has different meanings in film. In a screenwriting program, a "scene" is pretty much anything with its own slughead. That means establishing shots and little snippets within a montage. Those all get tallied in the scene count. I'm hoping you're not asking about THAT number because that number can fluctuate wildly depending on genre, writing style, and usage of montages. The pertinent definition of "scene" is the word that refers to a conversation, or a fistfight, or a car chase, or a prowling-through-a-home moment, etc. Each of these things will have several slugheads in it, but these things are all one scene each. Right? Now that this is clear...
Rather than answer how many scenes you should have, I want to introduce two separate questions:
1) How long should most scenes be?
2) How long should the entire movie be?
The answer to the first question is around 1.5 to 2.5 pages. And from there you can go up or down, depending on the nature of the scene. Yes, some scenes are perfect for a half page total! And some scenes deserve 8 full pages! Very true. But, in general, the default rhythm, call it the pulse of the audience, sits at around 2 pages. The key is that you write and you innately FEEL the scene should come to a climax, rather than looking down and seeing your page count requires it. When you get that innate feeling, you'll start to sell your work. The best way to acquire it is to watch movies and then read the scripts of those same movies.
As for how long a movie should be, the answer is roughly 90 to 120 pages. Beginners should aim for 90. Books will tell you 105 or 115 or even 120 but these bloated numbers will keep you lovingly unemployed. The sad rule of Hollywood is: NOBODY WANTS TO READ YOUR SHIT. So if you pleasantly come in with a nice compact script, you will have a much better chance of the readers actually reading the whole thing.
If you combine these two answers, you should get a rough idea of what your total scene count should be. If you just want to express yourself through film, you can ignore this entire post. But if you're trying to sell stuff, you gotta learn the quantifiable rhythms. And I should also state that if you're a Shane Black or an Akiva Goldsman, you can violate every convention there is, because producers will trust your name. But if you're a nobody and you want someone to give you $122,000 for a measly little PDF document, you'll need to deliver a familiar product to them. They're scared -- producers -- and they need a familiar rhythm to feel secure about purchasing your wares.
They'll also need an impactful story, but that's a separate thread.
Shanked