That's still not very many at all, but that is with a player-controlled army. Have you witnessed an AI army leaving a garrison, acting entirely without your involvement? If the same ratio holds, I think it's insufficient. Unless you're in a lopsided war, it should be safe to assume that if you can raise an army of size X then so can your opponent, and X will always beat X/10(ish). Not that garrisons should be able to fend off an army in an assault or endure long-term attrition, but the AI's decision-making between assaulting and forcing more attrition seems fairly reasonable in situations like that: a 10 to 1 assault is a safe bet, so go with the assault. Which, of course, means they leave a similarly-sized garrison there and the cycle repeats.
This could be addressed through the autoresolve imposing a penalty against the attackers during an assault if the defenders haven't been attrited significantly, representing the defenders' will to fight as their morale would be higher while they still have provisions and intact fortifications. Further, such a solution could be augmented by shifting the priority of offensive wars to capturing and holding bordering settlements rather than deep penetration of enemy territory and--in the player's case especially--decapitation (literal or metaphorical) of military leadership. I'm not sure about recent versions as I haven't played in a while, but the go-to strategy before was to capture as many lords as possible during a war so that--even accounting for escapes--you could reduce the number of enemy parties being fielded at any given time. This is a rather ideal situation in real warfare, and it was almost never the case that one side in a conflict would kill or capture the vast majority of their enemy's command without also winning the war unconditionally at the same time.
Therefore, I think the priority for AI should be to focus on a realistic, achievable goal in their wars, and once they have attained that goal they should take reasonable steps to secure it against recapture. Once taking their goal their priority should shift from initiating sieges (especially city sieges) to raiding and defending their recently captured settlements.
I'll make a thread for it, since I just wrote a description of a system for it and it was longer than the above.