Note! This mission is part of a series of quests that you can receive from Delvin Mallory at The Ragged Flagon. Completing them isn't necessary, though it will improve the Guild's financial situation and give you additional City Influence missions.
All The Numbers Job missions have the same objective: putting changes in ledgers. Each time Delvin will order you to reach a certain location (Markart, Whiterun, Windhelm or Solitude) and false given positions in the books. Afterwards return to the quest giver and collect your prize.
You can repeat this mission as many times as you like - further objectives will be randomly chosen and there's no limit to them. Just remember not to kill anyone while you're at it, as it will deprive you of any prize and additionally you will have to pay a fine.