I think Ri Boku wins this, but I think you guys are actually downplaying Ou Sen and his army quite a bit lol.
Let's go through this thing matchup by matchup and let me explain why I think this is a very close fight.
First off, even when Ri Boku had 200,000 and Ou Sen only had 165, that difference in numbers is negligible unless Ri Boku chose to fight specifically with attrition warfare, and even then he is not guaranteed a victory.
Second, let's look at these commanders in terms of overall ability:
Riboku >= Ousen
Akou >= Bananji
Makou (?) >= Shun Sui Ju (at least I would bet on it lol)
Earl Rai and Earl Kou ~ Den Ri Mi, Shi Ryou, and Sou'Ou
And in terms of overall ability on the battlefield...kinda tough to say but...I would personally argue Ouhon ~ Houken overall.
I won't elaborate on any of these here for brevity sake, so let us move on.
So overall, these commanders are pretty equally matched, and plus, and this is something nobody has commented on,
Ou Sen debatably has a terrain advantage with access to the exact same mountain fortress that forced Renpa himself into a standstill.
How would Ri Boku react to this? Would it have any effect on him at all? Or would it completely turn the tide in Ousen's favor? Extremely difficult to say.
Plus another thing to think about, and this is a sentiment I echoed above, Hou Ken is not a General. In fact, Houken seems to actually not have too many uses on the battlefield, for one he doesn't always listen to Riboku, and for another thing, he's a one trick pony who essentially can only do one thing very well, which is show an overwhelming amount of martial strength (for one guy lol). He's a very different tool than someone like Akou who knows numerous formations and tactics in addition to his strong martial strength and overall strong battlefield knowledge.
Overall, I do think Riboku wins this, I think his tactical superiority over Ousen as well as his access to Hou Ken as his personal OP ass body guard and occasional 1 man General slayer would be too much for Ousen in this particular scenario, but I think this matchup is one of the closest we have yet to see in the tournament.
The Riboku army wins, extreme diff.