How much data do I need to forecast?

Different methods require different amounts of data to forecast. In general, you probably should have at least 6 points of data (the minimum for Double Moving Average), you should have at least the number of historical data points you want to forecast, and the more points you have, the more accurate the forecast (to some extent).

Seasonal methods require two seasons worth of data. For example, if your seasonality is 12 months, you should use at least 24 months worth of data.

Regression requires historical data equal to 3 times the number of independent variables.