When your publisher sets the print run for your book, they will typically take into account a number of complex factors—often right up to the point where they place the order with a printer. They will consider backorders from the major retail (Amazon, B&N, etc.) and wholesale (Ingram, Baker & Taylor, etc.) customers. They may also consider media interest—especially from high-profile library/institutional media in the case of scholarly books. They may also consider any special sales or sub-rights/book club sales that might have been arranged pre-publication. Because every book and every publishing house is different, you should ask your publisher directly if you have specific questions about your book’s print run.