The numbers hand-written on lithographs represent the order in which the artwork was printed and how many total prints were produced. For example, the numbers "58/100" written on a print means that particular lithograph was the 58th print made out of 100 total prints.
This practice did not come into being until the late 19th century when modern print-making techniques allowed artists to produce higher quantities of prints. Some artists began including these numbers on their prints so that buyers would know that there was a limited quantity of prints available. This makes the prints more valuable or desirable to some collectors. This is also done for other types of printed art, such as copper etchings and wooden block prints.