i also hate that they make armies in peace time, not because i cant collect everyone, but because the armies often go hungry and are unprepared when wars start. it also keeps the parties within the army from recruiting more troops because they only visits towns, often times the same one, and they cant be let free to chase looters infesting the country, among other issues, overall penalizing the economy
with that being said, you only need a couple guys in your army to be able to conquer and defend things. 2000 men is roughly enough to pretty much conquer anything. just 1000-1200 will usually cut it for any well garrisoned castle, even less if the owners of the castle are cutting costs and have removed most of the garrison
it is not the amount of parties in your army that matter or being able to summon everyone, its the size of those parties that matter. you can do with very few but large parties and be much more efficient with your influence spending. having too many parties in your army drains your cohesion a lot faster no matter of how many men each party has, so you end up spending way more influence even if you also summon all your companions than if you just grabbed whatever few guys with +100 men in their party. managing an army with around 15-20 parties in it means you have to refill your cohesion daily because it drops below half every day, or even less time. if you lack more guys, its better to dismiss those that have too few men in their party now and summon new in, now that other armies are dispersing, than it is to invite all in at once from the beginning and have them all drain your cohesion from the start for little other reason than being big on the world map
unless your faction is on the verge of dying out and is losing all its clans - it does not appear to be - and with that guy on your screen only having summoned 9 parties, you should still have way more people to summon than he has, enough to be able to take maximized towns. but you still dont have to summon everyone to make success out of it
another reason why its a bad idea to have every party under command is that no one are present to defend villages. the more villages that get raided, the more the tribute for peace rises. taking towns and castles instead of defending raises the tribute too. defeating enemy armies raises the tribute you receive. letting parties roam freely to raid enemy villages raises it in your favor, too. making your enemies bankrupt doing peace has a stronger impact than taking a few of their fiefs. you dont need a lot of parties under your command to defend fiefs if you break into the sieges, and the death toll for doing that will be way smaller than facing them on the open field even if they are outnumbered there. and once the kingdom tab shows you have the upper hand in units and prisoners captured, you wont need a lot of parties to go safely on the offensive, just enough to meet the garrison size... because no one will be around to stop you