50# means 50 lb. sheets, which is how heavy a stack of that kind of paper would weigh. I don't think there's an actual standard for how many sheets of paper are in a "stack," but...yeah. In general, the higher the number, the thicker the paper.
Are you asking about the paper that it's printed on, or the paper that artists actually work on? I've seen paper that's supposedly used by comic book artists, with blue gridlines or something..but they come in very small packs and they're expensive. Inking on smooth bristol board would work just as well, I think....