That depends entirely on the size (volume, actually) of the pieces you
happen to make.
Also, there are various claims for PMC (and Art Clay Silver) that range from 10% shrinkage to 30% shrinkage so you'd have to subtract that volume from whatever pieces you have in mind (those are older numbers though so may be different now, and also may be for only the original
forms of each brand). Metal clays are expensive though, not even counting the cost of firing.
You might want to check out the PMC site or one of the groups associated with PMC or other metal clays to get more specific info... there are some links at least on this page of my (polymer clay) site:http://www.glassattic.com/polymer/Characteristics.htm
...click on category near bottom of list called Metal Clays
Oh, and I forgot that there's actually a "sticky" thread here named Precious Metal Clay
... that would be the best place here at Craftster to ask this question. You can always find that sticky thread/topic at the top of the list of threads/topics of the Trinkets and Jewelry: Discussion and Questions
sub-board (it will be on of the ones in gray
HTH at least some,