Bethesda may have shipped 12 million copies of Fallout 4 worth a huge $750 million, but what about Steam sales? According to Steamspy's stats, over 2 million copies of Fallout 4 have been sold on Steam so far.
If the sales are this strong this early on, we should only expect them to continue to rise. It has only been just over a week since Fallout 4's release, so where do you think it'll be in another 2-3 weeks? We could be looking at 3-4 million sold on Steam by then.