I don't get why everyone keeps saying there weren't, why don't we use our big brains and figure it out-
If you look at the SMAC stations in H2, they are only about roughly 5-10,000 ft. tall and maybe 3,000 wide.
When on Reach, you cannot see them, of course, that would be like claiming the ISS and hundreds of satellites aren't real because you can't see it from Earth. In LNoS, the only mission you could POSSIBLY see them in, you are most likely far enough from reach that you can't see them (Reach is a big planet, trying to see them from Anchor 9 would be like trying to see spacecraft orbiting Earth from the Moon) or they may be farther away from Reach.
Why didn't they fire on the Supercarrier or Corvette?
Perhaps the Covenant Supercarrier was sending some type of jamming signal when it was on Reach or when in space that disabled the MACs, similar to the plasma pistol's EMP. Also, who's to say a MAC could destroy a supercarrier in 1 shot? In TFoR a supercarrier is capable of slicing ships clean in half with it's blue energy projector, and maybe the UNSC thought it would be smarter to use the slipspace bomb. You know, a cleaner kill.
Also, the Corvette in ONI: Sword base is shot with an SMAC round, so that instantly invalidates your arguments anyways