Orbital bombardment could probably be carried out with relatively few ships, I would think. I mean, look at the devastation a single nuke can do, in our world--just imagine the yield of a quantum torpedo.
(Also, as an example, remember the Xindi attack on Earth in the 22nd century.)
Would have loved to see that on screen when Sisko orders the Dominion fleet put on screen you see the main force arrayed out to face them and then in the background you see ships firing on the surface and noticeable explosions.
As for how many planets were affected it would also depend on how many planets in the Union had a Jem'Hadar or Breen presence and whether or not the Founder's orders to kill the Cardassians reached these other garrisons before the Cardassian military started to actively turn on their "allies".