sales forecasting for multiple merchants

[amroghoneim]

Hello!

I have monthly sales data for multiple merchants using our online payment service.
I want to forecast next month's sales per merchant.
What I have tried to do so far is to come up with features related to time of year/quarter/holidays as well moving averages.
I try these features along with the linear component.
The results are satisfying for some merchants and the complete opposite for others, where I see cases where the model overshoots the actual month sales by a considerable margin, I see it also sometimes failing to capture the seasonality.

What is the best way to approach this problem? and what is the recommended minimum number of months I should have for good forecasting given monthly data?