The Cardassian military was organised into a home defence force and an exterior 'Guard'. The guard, along with various militias, are responsible for policing the borders of the Empire, and latterly used by the Dominion as fleets for the invasion of the Alpha Quadrant.
The guard is arranged into at least twelve 'Orders', with each possessing ships (each with its own Gul, roughly equivalent to the rank of a Captain in Starfleet), space stations, bases and armies, and held under the command of a Legate (roughly equivalent to the rank of a Starfleet Admiral) or a senior Gul.
Each order appears to be responsible for controlling a specific area of space. For example, according to DS9: Emissary, the area around Bajor is controlled by the Seventh Order.
JASAD: I am Gul Jasad of the Cardassian Guard, Seventh Order. Where is our warship?
and the Fourth Order is relatively close by.
CARDASSIAN OFFICER #3: Why risk a confrontation? The Fourth Order can be here in a day...
Dukat is, for a while at least, in command of the Second Order which seems to cover the area in which the Maquis operates.
GUL DUKAT: (addressing Cardassian ships) To the pilots of the Cardassian attack vessels. This is Gul Dukat, Commander of the Second order. Please respond.
The number of the order doesn't appear to relate to the size of the order, nor to its relative importance and may simply relate to its location in relation to Cardassia Prime or the date of its creation. The 11th Order, for example, controls multiple planets and an army of over half a million troops.
In addition to the twelve military orders (numbered one through twelve), there was also a secret police force known as the Obsidian Order who were able to operate with impunity throughout the Union.