10,000 Knights? Garland is flicking his finger in what he's using in the Reach's power. Remember though...in an era like that, people starve easy. He doesn't want to deploy men needlessly. The Knights of the Reach are also fucking landholders too, they have peasants who work the wheat fields and make bread to sell in markets. He's already sent 10,000 into King's Landing. He has another 10,000 on the borderlands, and another 12,000 he's sending to KL with his own forces. That's already 32,000 men we have accounted for, just under half. If the Reach wanted to, it COULD send another 10,000 to the Stormlands, and actually, this is a viable move later on. Right now? Peasants collecting grain. But a good number of mercenaries are coming from Yunkai, not just the raiders off the Westerlands, an actual force that could be deployed immediately to King's Landing, in number, though they're weeks out. It's soft power, it's careful maneuvering of the pieces. All it would take is one provocation, and bang, there would be no excuse, there's almost 90,000 that could be deployed in retaliation, not just in one place, but spread like a fucking scourge, and then the Stormlands will be the last thing on Garland's mind. 

And anyway, Garland's letter is frank as fuck, tbh. He doesn't pull all the punches, but it will be a message that says "Please, please, just pull your head out of your ass, fight these people, and I'll give you everything you ask for, in return for an alliance." So the way Gris responds decides whether he basically ends up with his head on a pike or being forced to do shit he really, really does not like doing, also involving Tarthmen, to keep in Garland's good books.