Dragon Age Inquisition, the last big AAA RPG, sold about 1,15 million units worldwide on its first week. Now it's standing on a total of 3,37 million acording to vgchartz
http://www.vgchartz.com/gamedb/?name=dragon+age:+inquisition
Skyrim on the other hand sold 3,4 million on the first two days (physical) and sent about 7 million to retailers on the first week. Now it's sales stands close to 20 million units (or 18,17 million acording to Vgchartz).
Beating Skyrim is almost impossible. It was launched for a big variety of plataforms and Bethesda with their Elder Scrools games have more years and more fans on the game scene. But, I think it is very possible to pass Dragon Age Inquisition numbers. For some motives:
The WItcher 3 will have modding support and is a open world game. This will get the attention from a great variety of people, inclueded DA, Skyrim and other RPG fan bases.
Second, it's been called as a Skyrim killer. It will (it's getting) the curiosity of gamers, even non rpg ones. Also, the fact that it's also been called as Game of Thrones equivalent, will help get gamers and non gamers attention.
Third, Bioware had the problems with Mass Effect 3 (cough, the ending, cough) which were quite a negative press for them. CD Projekt RED has the Free DLCs and Non DRM policy to their favor.
I think we can think of number of 2 million on the first weeks and maybe 4 million with some months, worldwide, but we can also expect bigger numbers. As you see, The Witcher Series sold 7 million units worldwide since 2007. With we estimate that 1/10 that Skyrim players will buy The Witcher 3, we have 1,8 million players. Doing a very rough math, we have an estimate public of 8,8 million players.
Ths without considering non Witcher or non Skyrim players (or even if more than 1/10 of Skyrim players decide to buy the game). But, as a word of caution, it's a very rough estimate. We can't say that we have 8,8 million unique players. But it's the best I can do with the data at hand.
I resume, we can safely bet a 1-2 million on the first weeks and 3 to 4 million on the following months (worldwide, physical and digital). But the numbers can arrive at more than 8 million. It has this potential.
Now all we can do is wait and hope for the best