There are roughly 34,000 Orlanth initiates in Sartar (including the Far Place), 5500 Yelmalio initiates, and another 8000 initiates of various war gods (Storm Bull, Humakt, Maran Gor, Babeester Gor, etc.). Let’s remove the Yelmalio cultists and say there is a pool of about 40,000 people who could theoretically be called up for military service by the Prince of Sartar. That’s the whole pool, including the elderly, farmers, herders, you name it, out of a total population of around 180,000 people.
At the Battle of Dangerford, there were about 5500 Sartarites present – that’s getting near 15% of the total military pool. And roughly the same fought the following year at the Battle of the Queens. That’s getting us into the sort of mobilisation that Alexander the Great had for the invasion of Persia. And you have to assume that still others are mobilised to serve as city garrisons, other tribal militia, etc. That’s probably the most that could be mobilised for any length of time without harming the kingdom’s economic and agricultural base.
Now Argrath brings another pool of 5000 or so Praxians (including 1500 Pol-Joni) sworn personally to him and, as nomads, are always mobilised. With that you can see that he came to the Principality with his own significant power base.
And it is one more good reason why you ought to come to terms with the new Prince. You can join him and plunder foreign lands, or you can oppose him and have Praxian herds graze it what used to be your pastures and fields.
Feeding them is certainly doable – there’s a lot of open grasslands in the Donalf Flats (which is bigger and richer than the Better Place). There’s also a lot in Volsaxiland and Heortland. There’s a lot of activity in 1627-1628 as Argrath tries to consolidate his position as quickly as possible (also early on he reaches out to Nochet, which is likely also a resources thing).
How does the Lunar army mobilize and support itself? It changes a lot year by year! In 1621, the Lunar Heartlands Army had 64,000 soldiers mustered, with another 14,000 mustered for the Provincial Army. 2500 were in Dragon Pass, 2500 were in Prax, and 20,000 were in the Holy Country.In 1625 after the Dragonrise, there is nothing in the Holy Country or Prax, and only about 5,000 in Dragon Pass.