Complex sentence: Dime novels were issued in twos or sometimes fours, so a first printing does not list more than three numbers beyond the number on the cover, whereas a later printing may list a hundred titles beyond the cover number.
Split into simpler sentences: Dime novels were issued in twos or sometimes fours, so a first printing does not list more than three numbers beyond the number on the cover. However, a later printing might list a hundred titles beyond the cover number.