How many men would you say the Lannisters can field, circa ADWD epilogue?

Well, the Lannisters started the war with 35,000 men under arms, and raised another 6,000 after the Battle of the Camps, which brought them pretty close to their maximum. I would say the Westerlands can raise ~45,000 men, 50,000 if you stripped the country bare of garrisons and the like.

Of Jaime’s 15,000 men, 2,000 are lost at the Whispering Wood, 8,000 are lost at the Battle of the Camps, and when Tywin calls up a new host at Oxcross with the 4,000 survivors of the Camps plus around 6,000 green men, the remnants of this original army are wiped out.

Of Tywin’s initial 20,000 men, an unknown number (but substantially more than 500) are lost at the Battle of Green Fork and the retreat to the crossroads. Perhaps a few hundred are lost during the reaving of the Riverlands, and the back-and-forth fighting at Stone Hedge, Darry, Maidenpool, etc. Of the remainder of the 20,000 who made it to Harrenhal, an unknown (but quite substantial) number of men are lost at the Battle of the Fords, another unknown number are lost at the Blackwater, another unknown (but “heavy”) number are lost at Duskendale, and an unknown but probably light number are lost at the Ruby Ford. 

I would guess that, following the Red Wedding, the Lannisters  had ~12,000 men under arms with Tywin, but definitely fewer than 15,000. Of those men, 800 go with Jaime to the Riverlands, another 2,000 are sent to assault Dragonstone (with 1,000 dying in the process), and the rest demobilize. In addition, Daven Lannister has around 1,2000 men that he scraped together from the survivors of Oxcross and various garrisons to besiege Riverrun. 

So at the moment, the Lannisters have around 3,000 men currently under arms, and perhaps 9,000 men in the Westerlands who they might be able to call on. I say might, because after three years of hard fighting, most of the Westerlands houses would consider their feudal duties more than satisfied and would probably not answer the call unless there was some existential threat to the Westerlands themselves. Especially with the rulership of the West being both remote and somewhat in question.