The document you're viewing (the man page) has 99 lines. If you scroll to the bottom of the document, the 99th line will be at the bottom of the terminal. The pager (less) reports the line number at the top of the terminal: when you're at the top of the document, you'll see
line 1/99; at the bottom, you see
line 52/99 because you have a 49-line terminal (showing the last 48 lines, i.e. 52 to 99, plus the less status line).
Less doesn't normally scroll beyond the last line of the document. But if you explicitly tell it to (e.g. with
53g to put the 53rd line at the top of the screen), it adds empty lines (actually, lines with a
~, unless you run