If you have your own castle or town, put them away when you don't need them, especially during peacetime. There's no need to have that many troops when you're running around doing patrols against bandits and whatnot as a small token force (size being dependent on difficulty) is enough to deal with that. And, even though it doesn't really affect the game too much (though this can be applied with your troops to a small degree), it's something I keep in mind with my own game:
"A Guardian is a symbol of comfort, but an army is a symbol of conflict. ...If this is the size of our defenses, what is it we're expecting to fight?"
-Ozpin (RWBY)
During wartime, you'll still want to be careful about your overall troop size as well since your renown will drop, IIRC, every week by a certain amount. I'll get into what I remember later on that subject. I wouldn't take a full cache of troops with me unless I'm doing a solo siege and in a location that I can get to fairly easily (say within a day). And even with the idea of a solo siege, I wouldn't even take a full set of troops either as I just need to scout out what I'm looking at, get the appropriate troops (and hope I'm not wrong on what kind of defenses the location has in terms of castle layout), and then siege. After doing that, go straight for your castle/town and unload what you don't need.
If you're doing more defensive fighting, you'll definitely want to have less than your maximum troop size to make sure desertion doesn't happen because you lost some renown and, if the amount lost was high enough, lose troops. Being on the defensive will mean more of you running towards a war party to either ward them off and hopefully have your presence keep them away as you wait around there for some time or be able to fight them off with those troops. If, however, they have a large quantity of troops, you'll have to take appropriate actions depending on the situation. Is it a village they're raiding? Is it a town/castle? Do you have enough troops and confidence in your party to be able to handle the situation or should you try to call reinforcements or, worse, abandon it altogether?
As for the renown thing, you gain a certain amount of renown per battle (default being a maximum of 50 per battle I think, but those tend to be rare unless you're vastly outnumbered IIRC). Unlike something like Honor, you lose Renown overtime regardless if you're at war or not (I think it's per week you lose a certain amount). There's also a bit of a "soft cap" on renown that, once you get to a certain amount of renown, it's too difficult to keep up with its upkeep as the more renown you have, the more renown you can lose per week; thus you end up in what's essentially a "soft cap" on renown so as to make sure you don't have some stupidly powerful army of 600 because you have infinite amounts of renown. There's also the fact that you'll gain either very little or no renown after battle against things like bandits or small war parties, so keep that in mind as well.
Hopefully this is helpful. If not, then someone with better knowledge might be able to help.
