I went in with a similar expectation actually. Would be interesting to have an idea of how much light'd fall on you if you were standing under the storm.
I imagine you wouldn't really enjoy the sight whilst being under it though!
In case you didn't read it, the "What if?" book has a couple pages dedicated to this exact question (not 5000, but "how many would it take to block the sun?")
Sounds like a great candidate for a What If submission.
Speaking of, we haven't had a new What If in nearly a year now.
Here is my speculation: you cannot fit enough archers in the relatively small area from where they can still shoot at the same target and cover a significant portion of the sky with their arrows.
Unless the flight paths converge at the end.
When the arrow hits your face, you can no longer see the sun. Therefore one archer is sufficient to blot out the sun.