We are to complete an PICO assignment which is going well but I need help with completing the flow chart. I attached my Professor's flow chart example and other examples he has given us. What confuse

We are to complete an PICO assignment which is going well but I need help with completing the flow chart.  I attached my Professor's flow chart example and other examples he has given us. What confuses me is how are people able to determine  (N) just by looking at an article. How are they finding these numbers for (N)? I have also attached PDF websites where I will be using as my evidence for my PICO research assignment.  Please show me how to find these information. 

In attachment 3 is one of the articles I am using in my evidence paper. How am I to find these numbers for (N). 

Please help because I am lost