Right, so in the hypothetical battle envisioned by Sam Stone, both armies lose half of their combatants. The NK’s army of wights goes from 50K to 25K. The 25,000 wights that are “killed” by dragonglass, Valyrian steel, or fire stay dead.
However, the army of the living also sees its numbers reduced from 50K to 25K, and the 25K that are killed are immediately raised as wights, so it’s actually now 50K wights against the remaining 25K living.
Regardless, in such a battle, there’s really no way for the living to prevail in a normal straight-up fight. The only way to win is to take out the Night King and the other White Walkers.