Vertical Convergence is a number from 0 (top of the usable section of the page) to 1, which is set to three times the height of the usable section of the page, and creates nearly parallel lines.
Number of Perspective Lines, is the number of lines that are calculated. Depending on your setting - not all of them may be drawn. If you want more lines... just bump up the number a bit. This is not an absolute.
Depending on the settings, there may be blank areas near the corners of the document. This is because the number of lines that would be required to fill this area would get out-of-hand.