Why did they title the book number the stars

The title "number the stars" refers to psalm 147:4 in the Bible. Peter says this to comfort people who are afraid:

O praise the Lord.
How good it is to sing psalms to our God!
How pleasant to praise him!
The Lord is rebuilding Jerusalem;
he gathers in the scattered sons of Israel.
It is he who heals the broken in spirit
and binds up their wounds,
he who numbers the stars one by one… (10.34)

Peter is alluding to the idea that God can see everything, he can even count the stars, and knows their suffering. The book alludes to God seeing everything and has, through many good characters, the ability to ease the suffering of persecuted Jews.

The title "Number the Stars" is also in reference to the promise God makes to Abram (Abraham) in Genesis 15 that his descendants will be as numerous as the stars. These descendants are the Jewish people. "And he brought him outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your offspring be.” (Genesis 15:5, English Standard Version)

You could read that as a promise that the Jewish people will not be annihilated as the Lord promised that Abraham's descendants would be as numerous as the stars. It's a promise that no matter how horrible the Holocaust might be, the Jewish people will survive.