The non-obvious observation, but the observation Wielki Ksiaze has nonetheless made, would involve putting numbers on things.
The armies of Odranto have between between three and four thousand professionals, plus baronial retinues. The armies of Lastwall are deployed on three separate fronts, neither of which is with him; the troops they have bordering Ardis are - perhaps five or six hundred, scattered across sixty miles of border, and a town guard in Chastel that will also number in the hundreds, with a militia well under a thousand; between Ardis and the verdant fields of Canterwall and southwest Lozeri are thirty miles of swamp and forest broken only by thin streams. The forces of Lastwall are still mobilizing. Sinaria, no doubt, has its own host, but though their elite companies of sorcerers are of great value in war, Ksiaze would guess their full numbers at under two thousand; the old structure of the counts that once was has been destroyed, and the Red Witch has not yet rebuilt her forces. Any militias or town guards she calls together will be under a thousand, for Sinaria has never been a warlike province. And the Order of the Pyre have no more than three hundred Hellknights and a thousand armigers, and the same number of Barstoi militiamen; hardly a threat to the Grand Prince.
And what, pray tell, does Ksiaze have? Ksiaze has client necromancers, and scattered lesser allies of the Way who have placed their thralls under his command, and undying beasts of war... and four hundred years of skeletal champions.
A reasonably competent crafting wizard may make four hundred gold a week; one gifted enough, as Ksiaze and his underliches are, to reach the point they do not need to be careful about every petty detail can make a good deal more than that. There are upkeep costs, yes... but bodies are not expensive. The main cost of his graverobbing expeditions is the Scry and the two Teleports required to collect the bodies, and he made a crystal ball to handle the scrying for him generations ago. He pays his graverobbing operations well enough to let them drive out competition, and yet, in actual fact, he still spends less than four thousand gold per corpse he obtains.
And, frankly, the costs of maintaining these stealthy operations are the main limit on how many minions he can obtain, not how many Evil adventurers die in Golarion. If all else fails, he can just set up in another city, somewhere else in the world. It's a big world!
Or, in other words, he has spent the past four hundred years raising more than sixty thousand skeletal champions.
Now, his numbers are not quite so good as you might think. Some broke under the strain of undeath and went mad; some refused to serve and had to be put down, some were useless after their time in Hell or, for that matter, destroyed in his recent battles. But the actual core of his force is terrifyingly immense, for the graves of the Hellknights are filled with the bodies of random peasants whose corpses were easy to swap with their fallen paralictors.
But he still has the largest army in Ustalav.
And the mightiest.
But that's not necessarily something that anyone else knows, especially if they assume he has some technique for controlling very large number of skeletons, which is really more plausible than him having a technique to control very large numbers of skeletal champions.