How do you forecast the materials for multiple items at once?

Our standard lead time from PO receiving to delivery is 3 months (although this could
vary due to few factors such as PO volume, customer requests, capacity restrictions,
etc.). The 3-month lead time is inclusive of 2 months material procurement time and 1
month processing time. This standard is communicated to customers so they can
arrange PO and forecast information accordingly in consideration of this information.