We do not require a specific amount to be kept in our warehouse. If the product is one that you provide (i.e. a book) then we suggest you keep a minimum amount on hand equal to or greater than the amount of sales you expect to receive in the time that it takes you to replenish your inventory. That means 30 days if it takes that long to print and get more books to us.
If the product is one we produce for you the amount we suggest you keep on hand should be equal to or greater than the amount of sales you would expect to receive during any average 10 business days. This allows ample time to reproduce that product for you before inventory levels reach zero.