With Great Xia's strength, it was enough to support these soldiers and horses. They could also do other things.
Where did the silver come from?
Salt, tea, silk, porcelain, and the Academy were just ordinary industries. Although the profits they earned were abundant, they could not support so many soldiers and let the imperial court do other things.
They were all supported by their pillar industries, which were controlled by the imperial court. There were spirit herbs, pills, ores, materials, and so on. The biggest income was still demons, monsters, ferocious beasts, and True Spirits. Any True Spirit caught was full of treasures. Blood essence, claws, eyes, scales, and so on. Every single one of them was priceless, and they could earn a huge amount of silver.
The army of the Shang Dynasty was similar. It could only be said that the dynasty was the only one who was rich and powerful. It was far from what small countries and small clans could compare to.