I think forty thousand Dothraki could cleanse Pelennor Fields of its movie Orcs. Instead of six thousand Rohirrim, it'd be a force almost seven times its numbers. The Dothraki, despite their lack of armor, were able to break the Lannister/Tarly Shieldwall in the Battle of the Goldroad independent of Drogon functions as an AC-130 overhead. Granted the shieldwall was smaller and thinner, but the ones that the Rohirrim crashed into broke immediately and the spears the Orcs had were way more flimsy and the shields far less mighty, as were the Orcs themselves by and large. The Tarly and Lannisters had better archers as well (though I appreciate LOTR's CGI pulldowns of one arrow killing a dude).
Plus some of the Dothraki were looking like pincushions, while other ones were firing from their bows while crouched atop the backs of their horses, somehow shooting the soldiers in the shieldwall in their f'ing throats. And riding through fire... which might be as impressive as riding their horses into a spearwall/shieldwall and oftentimes through it. Whatever mitigating factor the Rohirrim armor might have would likely be largely eclipsed by the fact the Dothraki are about seven times their number and the fear factor would be just as formidable, if not more so... and then multiplied by an even larger horde.
I think you could even add the Oliphaunts showing up and the Dothraki would probably do better. The Rohirrim's only reasonable option was to charge unfortunately and they did, and managed to bring a fair number down, but its notable they did so oftentimes through the use of archers and thrown spears and the like. The Dothraki would do better since they focus even more on horse archery and being extremely fast and mobile cavalry. Plus we've seen random Dothraki warriors fighting top tier characters in Game of Thrones, including the one who pwned Bronn on horseback in the Battle of the Goldroad. Considering the random crazy ass feats they were pulling off on the show rather casually, it wouldn't be hard to foresee similar shenanigans brought forth like how when Eowyn managed to singlehandedly cripple/kill an Oliphaunt herself.
So... yeah... open field... the movie Mordor Army. All of the Orcs and the Oliphaunt reinforcements that come in later. Not the Ringwraith squad obviously. There's no reasonable counter for that.
