Regarding forecasting in Tableau, I don't completely understand this theory given in Online Help
Tableau requires at least five data points in the time series to estimate a trend, and enough data points for at least two seasons or one season plus five periods to estimate seasonality. For example, at least nine data points are required to estimate a model with a four quarter seasonal cycle (4 + 5), and at least 24 to estimate a model with a twelve month seasonal cycle (2 * 12).
How can 2 seasons and 1 season + 5 period be equivalent. A season could be a quarter or 6 months. . In the above example 4+5 is 9 months and 2*12 is 24 months. the former is 2 seasons and 1 month while the later is just 2 season.