Krim here, going through your archives a bit and while responding to a question about generals in the Tyrell/Lannister alliance you stated the Lannisters only had 2,000 men in the Riverlands. I don’t recall the Lannisters demobilizing their forces before, so shouldn’t they still have Tywins original 20,000+ army under Jamie’s command in the Riverlands instead?

First, Tywin’s army was probably nowhere near 20,000 by the time of Tywin’s death. He’d taken casualties at the Green Fork, the retreat from the Green Fork, some casualties from raiding and counter-raiding, heavy casualties at the Fords, and some casualties at the Battle of Blackwater. I would estimate that of that 20,000 perhaps 12-15,000 were left. 

Of those, most left when Tywin died.