You have read 21 books from the ' crazy silly school ' series. If there are a total of 29 different books in the ' crazy silly school ' series How many more do you still have to read?
There are a total of 29 different books in the 'crazy silly school' series. 
You have read 21 books from the 'crazy silly school' series. 
That means you have read 21 out of 29 books. 
That means you still have 8 more books to read.