What does "She'ar Etz Yaro" refer to?
Rashi: The remnant of his soldiers. Radak - it uses this expression, for it compared his camp to a forest.
How much is "Mispar Yihyu"?
Rashi: A small amount 1 that are easy to count.. A child can write them. Sanhedrin 95b says that this is 10 - there is no child who cannot write a small Yud 2 , which signifies 10.
Sanhedrin 95b #2: Nine remained. "V'Nish'ar Bo Olelos... Shnayim Sheloshah Gargarim b'Rosh Amir Arba'ah Chamishah bi'Se'ifeha."
Sanhedrin 95b #3: Fourteen remained (we add all the numbers in "v'Nish'ar... Shnayim Sheloshah Gargarim b'Rosh Amir Arba'ah Chamishah bi'Se'ifeha." the above verse).
Sanhedrin 95b #4: Five remained - Sancheriv and his two sons, and Nebuchadnetzar and Nevuzaradan (who later brought the Churban) 3 .
Radak: This is like "va'Ani Mesei Mispar" (Bereishis 34:30).
Radak: When a child learns to write, he first learns Yud; it is the beginning of writing an Aleph.
95b: We have a tradition that Nevuzaradan was there. We learn Nebuchadnetzar from "v'Reveh Di Revi'a'ah Damei l'Var Elahin (his form resembles an angel)." Had he not seen the angel kill Sancheriv's camp, how would he know the form of angels?! We learn that Sancheriv and his two sons survived from "Hu Mishtachaveh Beis Nisroch Elohav v'Adramelech v'Sar'etzer Banav Hikuhu va'Cherev." (The verses do not say that his sons were there, but since he was so confident of victory, surely he took them.)
Why does it say "v'Na'ar Yichtevem"?
Refer to 10:19:2:1.
Radak: They are few, so a child can count and write them, without wearying.
Malbim: They are so few that a child can write their names.
