While opening with an action scene can work, unless the reader gets to know and care about the MC, then whatever is at stake for them in that opening is pointless. Asking a reader to inhale 3-6 characters in a very few pages is too much.
I've read the slush pile and done a lot of critiques, and names and descriptions don't stick in my head unless I have a reason to care about and identify with them. One action scene had so much action in it that the writer forgot to mention both the name and gender of the viewpoint character. Plenty of jumping about, running and shooting, but no hero!
It's been recommended you intro 3 at a time, then the next batch and so on, and I can agree with that. It really does depend how you structure their mission. If it is possible to accomplish their goal with as few people as possible, do so.
Please also tell us you've read plenty of non-fiction books on military operations. I have dipped into a few and what goes on is usually radically different from what's in movies and vid games.
Make absolutely sure all the names start with different letters and do not rhyme. No Jim, Tim, Lynn, Lane, Blaine or Caine.
Now a question: are you visualizing this as a movie in your head? That's a device I've used, but with fewer characters. Each gets their own intro as they occur in the plot.
But the problem with this device is having the narrative be
too much like a movie. It is a visual medium, and in a good film it is easy to follow the plot with the sound off. You are working in words, and descriptions of action do not take up much page space. What is unique to words is it allows the reader to get inside a character's head and feel their emotions and steam in their sweat. The character is letting them know what's going on internally. In a movie all we get is a closeup with an actor looking grim.
We don't need even a capsule description of all the characters, but focus on the viewpoint MC doing something.
EX:
Blake scowled at the innards of the bomb in front of him. "Gimme your knife, Jim."
Jim, face obscured by night vision goggles, was focused on the entry of the parking lot, alert for the next guard patrol. He slapped a pocket and pulled out a multi-tool. Somehow he dropped it into Blake's hand without looking down.
"This isn't a knife."
"Think outside the box. How long do you have?"
That was a telling question, how long do
you have, not
we. Anything less than a minute and Jim would bolt for cover. "Three minutes," said Blake, opening the multi-tool. The timer on the bomb had just passed thirty seconds.
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Okay, in that short bit we know Blake takes chances and Jim is not going to risk himself unnecessarily. We don't need to know what they look like, only that Blake had better resolve things in 29 seconds or it's the end of the book. Jim may give Blake hell for lying to him, or not, and if it's an important character conflict between them it can be developed through the book. In the next line a third member of the team can come up, see the timer ticking down, and sensibly bug out of there, or -- if he has absolute trust in Blake's ability to disarm bombs -- he might stick around. Which tells us something about
him.
The above bit could be a third person omniscient VP or third person subjective, depending how the scene proceeds. I prefer the latter, both to write and read since it projects the reader right into the MCs head. No one has to figure out who's the hero.
That's one of my other devices as a writer, make it clear from the start who the MC is or readers get frustrated. Dropping a dozen names into a few pages is just going to annoy them, but having to figure out who they're supposed to follow might cause them to close the book and move on to another.
While YOU may know each of the characters very well indeed, a reader does not. Ever been in a social situation where you meet a bunch of people in less than a minute? It feels like that. No one likes it much!