Sixteen.
That’s the number of lines of text appearing on the first page of a book. After the book jacket blurbs, it’s that first page that must compel the reader onward or the book is put back on the shelf.
There is a site that critiques those first lines; it’s called Flogging the Quill and it’s done by Ray Rhamey and commenters.
Two months ago I sent FtQ the opening to my WIP–Into Dark Woods. My opening is now up for the world to see. I can’t look quite yet.