Are your questions in regards to your own DB, or purchasing DBs from someone else's set? My understanding of this thread is that those of us who are ordering a cards mold set, and also want DBs are placing our art here. At some point when all the art is posted, (I'm assuming) James will open this up and those of us who want to buy other peoples buttons will indicate our interest, along with some DB collectors on the forum that are not part of the group buy. James will tabulate the numbers, collect money, and place the orders for the buttons to be made. He feels pretty comfortable that there will be enough interest in all the various buttons (unless they are very personal) that the minimums will be reached, leaving the owners of the sets only needing to buy however many buttons they want personally.
So from your other question, if you want 5 of your own buttons, it is pretty likely that at least 5 more will be purchased by other forum members, thus meeting the 10 per design minimum. If you are referring to other peoples buttons, I don't think there is a minimum. You can buy as few as one of any of the buttons that will be offered here in this thread. Just make it know here to James once he begins taking the orders.
@Himewad please correct me if anything I stated here is wrong.