How much do you have to show about the past of your characters in a flashback?
There are no rules. There's nothing you have to do. Flashbacks, though, they can be great, they can be a minefield. Let me talk about some of the minefield aspects of a flashback.
First is, you're gonna have to decide how you're gonna do your flashbacks, because there are a lot of different ways. I do my flashbacks in The Stormlight Archive as a separate narrative line and basically we have multiple timelines in the books, where you're getting a character's timeline catching them up to the start of The Way of Kings. This works very well in an epic fantasy because I have lots of space and I can separate these chapters off that are flashback chapters completely on their own and they can be isolated. More common, the type of flashbacks you'll see from a lot of people is the "stop and think about it" flashback, and then cut to a new scene and you are seeing actively what the character's remembering at that time. This is the Lost method, the TV show Lost, a lot of television shows and movies use this, and they actively show the character thinking about it. I rarely do this. Once in a while, in The Stormlight Archive, you'll see a character start to tell a story about their past, and I'll make it a line with the next flashback chapter that you're going to get, but really what's happening is a character's telling another person a really shortened version of events, because when you're getting the flashbacks, it's actually not a flashback, the characters thinking about it, that's a separate timeline.
Another way to do it is the kind of, in the middle of a chapter, you're not doing a scene break, you're just flashing back to what happened, and there it gets tricky with tense. Tense can be really a challenge with this. The "I had done this" or whatnot, or if you're not gonna use the tense, it can get really confusing if you're not gonna do a tense change, you're just gonna put that past tense too, both of which are viable, I've seen them done very well. But those in-scene flashbacks can get really "tell-y" and really hard for readers to track and kind of uninteresting for them to read. The danger with any flashback -- this is the one that has the most trouble -- is that the reader will feel like the story is not progressing and instead they're wasting time doing something else and that they're not interested in attaching to this. And this is a challenge even with The Stormlight Archive ones. There are people who just do not attach to the flashback sequences, because they are, by nature of their story, prequels. And that's a challenge of writing them.
What do you gain? Why would you do this? Well, it's a really cool way to build motivation for your character, depth for your character, and to show a different place and time in your story so that you can show how much has changed. We talk about show versus tell, and you can use a flashback to show a character's changes quite dramatically in this manner. You can also get information to the reader in ways that would've been really "tell-y" otherwise. Sometimes, flashbacks are really "tell-y". And when I say "tell-y", they're boring, because they're infodumps and they're just giving you a whole bunch of information. A lot of times, if you do a flashback right, it feels more active and more interesting way to get the same information across to the reader, rather than having the character sit and explain about their lives to people, you just get to experience and see it. But that's just one tool, right, and like all of them, gauge for your own story if this tool is going to enhance the story you're trying to tell or if it's something that you should save for a different story.