Though target numbers are used in all White Wolf games, the term was introduced in the Revised Storyteller System to differentiate target numbers from difficulties. In the original Storyteller System, the target number was referred to as the "difficulty" of a roll, and defaulted to 6, but this target number could be varied by the Storyteller or, in some cases, by specific rules. The Storyteller (or, again, specific rules) could also ask for a greater number of required successes. Since this gave two conflicting mechanics for representing the difficulty of a task, the Revised Storyteller System used a fixed target number (usually 7), which never changes. The variable target number was too entrenched in the existing rules for World of Darkness games, however, so this wasn't changed in the Revised Edition Storyteller games, which still referred to the target number as "difficulty".
The only Revised Storyteller System game in which the target number can change is Exalted, in which the basic "Excellence" Charms of the Sidereal Exalted reduce the target number for a given roll, rather than simply adding dice to a dice pool (as do the Excellence Charms of most other Exalts). This mechanic is unique, and was chosen to represent the fact that the Sidereal Exalted do not possess superhuman levels of ability, but rather bend fate to ensure the best possible outcome.
The target number in the Storytelling System is 8.