1.the need to encompass the uncertainty to give credibility to the analysis.if we do not cover all the possible alternatives to what may happen, the analysis may be faulty due to incompleteness.
2.the regularity of the impact of the variable.If the analysis is very sensitive to small changes in the scenarios are needed. If the results are not very sensitive,fewer scanarios will be needed to analyze the impact of the variable.
3.the owner's or manager's beliefs about the future or about the impact of variables will determine the number of scenarios needed. Some beliefs need to be tested and evaluated to see what impact they may have and whether that impact is important.As a result, some beliefs need to be changed.
4.the practicality of analysis will restrict the number of scenarios.More scenarios means more work needs to be done. Time is needed for both doing the analysis and interpreting the results. The comprehension and ability endurance of the intended audience also need to be considered.