In the episode before sacrifice Dukat stated he was pulling enough ships to crush them so I think Dukat saw how many ships were headed that way and pulled double the amount for them.
FWIW, conventional military wisdom has it that in 2D warfare involving "melee" weapons (be they swords, muskets or tank guns), you need to outnumber the defender 3:1 to triumph in attack. Nobody knows what would happen at 3D with "melee" weapons, but that's what Star Trek is all about - and geometry concerns would make it appear that the attacker is at a massive advantage
from the very start, as the defender may be in the entirely wrong place, not knowing the direction from which the attack comes. So a ratio more like 1:6 might be sufficient for the attacker unless he gets drawn into a long attrition fight.
What Dukat needs to crush his enemy would depend on such considerations. A 2:1 advantage to the defender would be fine if Dukat were defending a point target and geometry did not matter - but Dukat tries to stop the invaders at a distance from the wormhole (the Defiant
actually needs to go to warp to reach the wormhole after penetrating the Dominion line), so he's at the mercy of geometry all right, and might need to outnumber the Federation ten to one to stand any sort of a chance. If he only goes 2:1 and the Klingons suddenly come from another direction, he's at a massive disadvantage, even if the Klingon arrival only adjusts the ratio to something like 2:1.1.