I have played games where there were a hundred or so asteroids. all moving somewhat randomly, and I think they were fun.
You can randomly place rocks from the garden onto the board, then, at the end of each turn, each asteroid rolls two dice, one for direction, and one for distance. if they collide, they stop, if they hit a ship or fighters, they roll to hit.
simple mechanism, and it does take a little cooperation by all to make sure you move the asteroids, but it really doesn't take all that long to move everything each turn.
One scenario I play with that is "Attack the mining facilities" where the defender gets 6 randomly placed minig markers and the attacker has to "land" a flight of fighters on the asteroid simply touch the asteroid during the fighter movement and it's been checked).
That one goes over well at the conventions I play it at. I usually grab ships from the book, 3-4 per player.
my point is, the more asteroids the better, if you're needing to block LOS. I do like the GF way too though.