During Silverage, comic books consistently kept a height of 10 1/2 inches
(26.7 cm), their width narrowed from the Golden Age dimension of 7 3/4
inches (19.7 cm) to a Silver Age width of 7 1/8 inches (18.1 cm), then
widened to 7 1/4 inches (18.4 cm) in the 1970s and 1980s before
narrowing to 6 7/8 inches (17.5 cm) in the 1990s. Citation
My printer ReadyComics tells their artist exactly what they need to make your comic book print exactly how you want:
http://readycomics.com/pages/comicbooks.cfm
ReadyComics prints comic books in standard size (6.625"x10.25"),
manga size (5"x7.5") and ashcan size (5.5"x8.5"). Our default paper
stock for interior pages is 80# gloss text, and 100# gloss text
for the covers. We offer an option for sketch covers. Our pricing
structure is incentive based, meaning the more books you buy, the more
you save!
They also have templates avaliable so you know how much of your file is going to be in the bleed area/cut off area. Templates are free to download: http://readycomics.com/pages/techspecs.cfm
So, those dimensions in 300-400DPI would probably be fine, if you are looking for print. Number of panels is really up to you. Print it out in the dimensions above with just your home printer and see if the text is too hard to read, or the panels are too small then adjust from there. 
Sorry the quotes are copied strangely, my computer is acting up. 
