free offer in pharmaceutical wholesale

There is no GST on supply products without any consideration as the section has used the word with consideration, Still have a look at this article and have an overview on this matter

Link:/articles/free-supply-treatment-under-gst-provisions-30463.asp

Thus in your case as the supply is made for 12 items but the consideration is made only for 2 thus GST shall be levied only for those 10 items and not on the free supplies.