Finding Maximum Flow
Welcome to the forum. Attached is how I would do it. Note that there can be many different ways of reaching this solution.
The maximum flow is 37. I have colored the flows of different sections from entrance A, B and C in different colors which makes it easier to understand how the flow at exit is made up of. Let me know if any questions. Hope this helps.
Would you be able to help with how to improve the maximum flow of people?
Not sure if I understand the question well. If you are talking about improving maximum flow at exit then that is not possible without changing the network. With the current network maximum that can flow out is 37 at exit.
If you are talking about improving flows within network, it is possible to improve some flows by making use of circular networks inside like JKM and GDH such that it does not impact anything else. Have a look at attached where I have made suggestion in green in brackets as to what could have been done with JKM to improve maximum flow. Think of how you can do similar thing with GDH.