Samples are available all over the 'net, Google
how to write a comic script (if you haven't already). As the roughest rule of thumb guide, 6 panels (3 x 2) per page usually does the biz.
Tough to answer your "how many lines of dialogue" question. How small a font do you plan on using? How hard do you want your readers to work for their entertainment? Just for kicks I use Paint.net (freebie upgrade to MS Paint, with layers) to practice dialogue and narrator captions.
Looking at your favorite graphic novels is going to clue you in further. Take a peek at the expertly handled dialogue from the Girl Genius online comic:
http://www.girlgeniusonline.com/comic.php?date=20051007
...loads of info, but the reading eye is easily led through the dialogue sequence.
-Derek