As you stock and issue in Ea, then update both the field in the Item Master as Ea.
Now, define the conversion in the UOM conversion form as follows:
Responsibility: Inventory Superuser or Similar
Navigation: Setup -> Unit of Measure -> Conversions
Select the Intra-Class or Inter-Class tab as appropriate.
and here define the conversion of 1PKG = 12 Ea for the Item.
Thank you for the answer PS- By setting it up this way will inventory replensishment triggered in each- know to convert into a requisition with package- or do you have to manually udpate the UOM on the PO to pkg?
The primary Unit of measure flagged in the Item Master will default in the PO and you may change it based on your requirement. If you want Pkg to default always then setup the primary UOM as Pkg.