In this question, we know that there are 3 sections in weekly newsletter and they are all automatically selected by the Stack Exchange system. How does the system select questions?
From the official announcement of the newsletters:
- Hot questions for the week
- Random unanswered questions more than a few hours old and >0 score
Hot questions for the week most likely means the questions that appeared in the Hot Network questions list during the last week.
As for exact amount of each can't really tell, but until official reply would be given, giving the best I can.