Originally Posted by
Rhovanir
I don't think it matters too much in the show, since it's more character based than real political potential based, but for the sake of conversation:
The North is neutral but has a pretty limited army. I mean most of it marched South, fought for a few good years, probably a lot of loyal/good men/commanders got massacred at the Red Wedding , then the remaining men marched North and had a mini civil war in the battle of the bastards. The North itself is pretty tired. It got boosted by adding some wildlings to it but those are at the war by now.
The Vale threw it's lot with the North so these two regions are a packaged deal. The knights (safe too assume bulk of forces, maybe some armored footmen as well) are in the North, but I would assume the Vale could marshal another army of footmen/peasants from whoever is left in the vale. There's probably some vale tribes there as well but they don't really care. Either way you have Jon commanding one veteran but tired northern army, a valemen army of knights and potentially another army from the Vale.
Cersei has the Crownlands + whatever goldcloaks are in King's Landing (not a lot of troops to be honest but worth mentioning).
There's still at least one Lannister army (they lost at Casterly Rock and the ambush but probably still have a large army, well equipped and experienced due to all the fighting. Probably pretty tired as well but not as bad as the North).
The Riverlands bore the blunt of the conflicts with most able-bodied men dead. Whatever are left are probably just garrisoning stuff. The Freys may be dead but they did have a decent enough force. If they didn't scatter to the winds, Cersei could get them as additional fodder. Everything else in the Riverlands is irrelevant I'd say.
The Greyjoys are probably all fleet no army at this point yet still a good ally to have.
The Stormlands are a bit weird in the show. On the one part most men died either on the Blackwater or in the North for Stannis. However, as far as I remember from the books, they were pretty hardy people themselves, and a decent region. So you would assume some fighting men could still be there, and it's unlikely they would follow a Targaryen. Even more so, if popetion was still there after Stannis died (or perhaps as early as his defeat on the Blackwater), you'd assume they were conscripted to boost the Lannister force (Tommen was after all Baratheon in name, and it's not like there are Baratheon's alive as far as the average Westerossi Joe is concerned). So maybe another bulk of troops from there for Cersei.
The Reach is probably united behing Tarly at this point. If Tarly is dead by now, well that just means the Reach is in chaos, with lords scrambling for favors. Most would probably join Daenerys, but without a liege lord to rally them back into an army, I'd doubt they would contribute much. The last liege lord they had was Olenna, now the rulling family is dead so it's a power grab. The army Olenna gathered was presumably defeated at Highgarden so there isn't a standing reach army now. If Tarly is still alive, pretty sure he could whip out support against Daenerys and her Dothraki. I doubt he is the only Reachlander that hates barbarians.
Same goes for the Martells in the show. The entire rulling family is dead. Even worse, there isn't any important faction leader around (like Daenerys who is in the Reach and her very presence could theoretically convince lords to join her). Plus with the whole Dornish are crazy thing going, I'd assume lords would fight for power in the region by now. Uniting them would take too much time I'd say. And the main reason why the Sandsnakes were going South was to get the Dornish army up and going in the first place.
Daenerys by herself has an Unsullied army in the west, and a Dothraki Horde somewhere in the Northeast Reach/South of King's Landing. Not a big force, but enough to win in open field. Quite frankly Cersei could still win as long as Daenerys doesn't get reinforcements. She could wipe the unsullied out somehow, then garner more lords to her side as Dany's Dothraki grow more restless in the wating game. AND if you could fend off the dragons for long enough.
However, in the show most of this doesn't matter. My guess is Cersei's side has the bigger army to make it look like she has a chance but she's hopelessly outgunned by Dragons at this point. Maybe you get lucky and kill one or two if Dany goes all out against the capital, but that leaves two angry ones left. After all this war, Westeros is spent. If Dany attacked with big dragons at the beginning of the War of the Five Kings, she would have lost. There were enough armies, good commanders and enough morale to put up a fight. Now, she just has to not screw stuff up and not die. Difficult for Dany? Maybe. But odds are in her favor.