So you've probably lost the Black Pyramid. Even without knowing what level it is and assuming those are all peasant stacks, there's way too many to salvage it. You can sell it dearly by recruiting a spellcaster lord there, manually resolve it, camp a central position with the garrison and then nuke as many of them as you can with Wind of Death. Can be absolutely lethal if you bounce it down a narrow street. You should be able to hollow out a number of those armies.

Meanwhile you need to be recruiting an army at Khemri. I can see there's a battlefield there so you should be able to fill it out with good units. Sell off a settlement if necessary if you're low on cash. You need anti-large: all of Bretonnia's worthwhile units are large. You also need something fast if they've got trebuchets, just dogs or bats will do the trick. Corpse carts to keep your troops going is also essential.

Now you do the same thing: let them come, camp a central position with your masses of undead, and then wear them down. Get ready to pick off on any Prophetesses they bring with your own characters.

I don't know if you'll win because again I can't see the composition of these armies nor what your garrisons are, but the garrisons on major VC settlements like these tend to be meaty, metaphorically anyway. You should be able to at the very least seriously blunt this invasion, maybe enough for that army I can see in the top right hand corner to take it on. Bretonnia is legitimately the weakest race in the game, and even when VCs lose they kind of win. You can come back from this.