The ambushing army regrouped after the battle was over. Out of the 15,000 players that joined the ambush, only around 4,000 survived. Many of these players who died had also died during the first day of battle against Arther's army. These players had lost two levels and two Amulet of Rebirths, they decided to opt out of the war at this point. They were not even sure if the war contribution points they gained from these two battles were enough to cover those losses.
In the end, only 3,000 dead players returned. So they had lost another 8,000 players. The total remaining players in the reinforcement army had now reduced to 22,000.