Hi Vishal,
In order to get one proposal for all demands, you have to maintain "Lot Sizing method" as Periodic lot size for a product and planning area combination in product master data. According to your requirement you can maintain period type as week, Month etc. Maintain these settings and run MRP.
Also if product specification is assigned to product planning area , then system only considers lot size method as Lot-For-Lot.
Please check your settings .
Regards,
Kopal