Sometimes when you're writing a script, there are times when we want to know more about a characters background or see an event that happened prior to what happened before the story begins. Flashbacks, in my opinion, are nice as long as there is conflict and drives the plot forward. Depending on the film, sometimes flashbacks might be effective, and other times, not so much. If you're questioning if you need a flashback in your script, here's a few things to keep in mind.
What's the purpose?
As I mentioned, flashbacks, and scenes in general, need to serve a purpose in the story. If you just have a random flashback that has nothing to do with the story and doesn't serve a purpose, then you don't need it. Personally, I like to use flashbacks to give a bigger background of a characters background and give the audience an idea of who the character is so they can understand why they act the way they do in the present moment. Perhaps you have a character who isolates themselves from others and the flashback shows a traumatizing event that made them stray away from others.
Formatting a flashback
There's a few different ways you can format your flashback scenes. One thing you can do is end a scene with "BEGIN FLASHBACK" then put in the next scene and end the flashback with "END FLASHBACK" or put in the next scene heading and under it put "BACK TO SCENE"
Example: INT. HIGH SCHOOL - CLASSROOM - DAY
(Insert action and dialogue. Blah blah blah.)
EXT. ELEMENTARY SCHOOL - FIELD - DAY
(Insert action and dialogue. Blah blah blah)
INT. HIGH SCHOOL - CLASSROOM - DAY
BACK TO SCENE
(Continue dialogue and action.)
Another way you can format your flashback could be putting (FLASHBACK) in your scene heading.
Example: INT. GYM - DAY (FLASHBACK)
How many flashbacks is too much?
It all depends on the story you're trying to tell. For example, murder mystery films use flashbacks a lot because they're providing background on the characters and showing each characters backstory prior to the murder taking place. Just note, no matter what story you're telling, use your flashbacks wisely. Sometimes you might not even need to use a flashback and instead use subtext.