Look at The Stand (by Stephen King), Tolkien. Send your characters on a journey.
I haven't read the above in decades, but they have many characters (far more than eight) and you can follow their pattern: Characters are introduced as the story is developing.
You already say each incident is different. You need to link your incidents into a story line, that just ONE or TWO main "leading" characters encounter along the way. King has a "good team" and "evil team", introduce one guy at the opening of the book (intentionally very average) and begins his story. That's the good guy, and becomes the leader.
At first he strikes out on his own, following a dream (that all the good guys also have and follow), but along the way encounters more of the good team. Weak ones that need protection (he is a good guy so he 'adopts' them), etc, and is eventually seen as their leader.
King alternates with the bad guys, similarly. An incident, they set out, and without intending to do so, either create another incident or run into one that yields another bad guy.
Tolkien does the same. Characters are introduced throughout, as they are needed, they are met.
Make sure your characters NAMES are different enough to not be confused; no pairs the reader would get confused like "Jake / Jack" "Bill / Phil", or "Mary / Marnie", no matter how much you love the names. Baby names are available online, or get a character naming book, or use your imagination for invented names. Try to make them look different (not similar spelling) and sound different when spoken aloud. Neither spelling or sound should allow confusion. And unless you desperately need it for some mistaken identity plot reason, no two characters with the same name. It is a cliché route to comic effect, "Hi. I'm Larry. This is my brother Darryl and this is my other brother Darryl." (Bob Newhart, 1982)
To introduce a lot of characters, find a way to connect your inciting events in some sequence. Preferably chronological, perhaps in two geographic paths instead of one (one path for heroes, one for villains), and give the first characters, because of what happens to them (perhaps investigating what has happened to them) in Miami, reason to visit Dallas where the next incident and hero/villain has their transformative beginning: Reason for the first to witness this event, at least.
People will get bored with eight consecutive disconnected chapters of introduction. Two would be okay, a good side and an evil side, but IMO for eight: Your only good solution is to connect them through the POV of existing characters.
Note that POV does not necessarily have to join forces or meet up: Joe is introduced in Miami, travels to Dallas, witnesses the inexplicable destruction of Dallas. Joe's actions in Dallas (helping people, running away) do not have to result in any new allies, but after Joe decides to leave, or gets his Clue that he needs to get to Los Angeles, he leaves, passing the rubble of an airport which is described. Joe finds a sturdy bicycle there in good shape. Before he leaves, he spray paints on a still-standing wall, "Get To L.A." along with the date and "Joe". Then he mounts the bike and starts out on Interstate 10 for L.A.
The next chapter lets Joe go (in the reader's mind, Joe's trip can be uneventful for several chapters). Instead, we connect via that bicycle, not a quarter mile from where Joe departed, we find new character Kevin, trapped in the rubble and watching him leave, he wants to shout for help but cannot. He sees Joe paint something. Then Kevin's struggle to escape, eventually he does, and sees the sign, and he is off to L.A. too. The reader knows Kevin and Joe will meet up.
The point is, There is a story link in the reader's mind. It is the magic of touch or proximity, a tenuous link, but a continuous one by virtue of the common setting of the collapsed airport, and Kevin knows Joe exists, and has some reason to be headed for L.A. so ... why not? Until something changes his mind. Maybe Kevin finds a working motorcycle and catches up with Joe. If Joe is leaving behind spray painted signs, other new characters can encounter them too.
Of course that's an apocalyptic scenario, but similarly engineered ideas can apply without total destruction, and without magical dreams. Connect your characters, give them a mission in their chapter to get somewhere, build something, find something, etc, but make it a mundane mission that will take some time so the reader does not mind leaving that character to make some uneventful progress for a few chapters while they read about something more interesting than the interminable grind of riding a bike 800 miles.
This avoids flashback. An alternative that can be used on its own or in concert with this is simultaneous events in different locales. A chapter begins with a bold sub-heading (or on screen a title), like "Christmas Day, 2030, TOKYO", aliens arrive, a character is introduced. The next chapter: "Christmas Day, 2030, NEW YORK", the next, "Christmas Day, 2030, CAPETOWN", then "Christmas Day, 2030, SYDNEY", etc. These are not flashbacks (memories of a character), but a common method of describe simultaneous events, I've seen the time specified to the minute for such simultaneity. Which precludes describing them all at once in a jumble.
However, I prefer the "story linked" introductions myself. One other trick is a different kind of POV link: Character Joe is talking to people, looking for something or someone, and hears tell (via TV, radio, telephone or other remote means) of something happening in Seattle. He knows he must go there. Next chapter: Seattle! Linda's event! She leaves Seattle! Next chapter with Joe: On the way to Seattle, he meets Kevin. Next chapter with Linda: she meets Joe and Kevin.