In a turn of events that is in no way surprising, Fallout 4 has done quite well in the old sales department. Bethesda are out there today, tooting about 12 million copies being shipped to retailers (which actually doesn’t confirm sales figures, but suggests they anticipated a lot of demand).
For PC sales, we have a few tools to work with; namely, SteamSpy. That site estimates around 1.5 million Fallout 4 owners on PC, with 1.2 million or so having actually played the game. There’ll be some margin of error on those numbers, but they should be in the ball-park. A Boston ball-park, presumably.