In chapter 12, there were several references to stars. Why does author mentions stars? Why do you think the book is called 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.