Help with customizing statusline

[rmad17]

Feature requests or bugs exactly did not fit my requirements. Hence adding a different ticket.
I have been trying to customize the statusline following docs and its been quite helpful. However, I am so far unable to get the total number of lines count. How can I achieve the same?
P.S. I am new to Luascript, and I would be very grateful if you could share a few detailed steps on how to work on it.,