Why not deploy so many troops in the battle for Shu City? Because there isn't... the timing doesn't allow it, and that's the awkward situation for Qin Country.
They can only fill the subsequent troops into even larger gaps, and then be crushed by the Tang Army with even bigger millstones. And to maintain the front lines, they have to continue recruiting more troops to do the same thing.
The newly trained troops actually are not that many. There are hundreds of thousands of troops, but most of them are scattered, and the majority are still on the way, with only a few able to reach the front lines.
On the Qin Army position south of the entrance to Shu City, a general named Ying Qing received his commission and orders: as a member of the royal family, he was temporarily promoted and appointed as the Supreme Commander of the northern war zone of Shu Territory, commanding all front-line troops.
