The below will show you how to add a minimum order quantity for purchase orders at a supplier level E.g. a Purchase Order needs to be £500 or it needs to be x amount of units
There is the option of having individual items from a specific supplier have a Default Order Qty, this is set through [Stock Item | Suppliers], or you can set the items Minimum Order Qty and / or a default Order Qty from [Stock Item | Properties] (This setting will affect any supplier not stated in the stock item supplier list).
It is not possible to set a global minimum order qty / value against a supplier.