Companies don't need mega sales to make a sequel at all. If crysis 2 sold 5-6m i'm sure crytek would be extraordinarily happy. It won't be as successful as bad company 2 was though (about 8m copies or so), since that has a great pc fanbase supporting it (and even though it was a console spin off series, they supported the pc very well), and the console fanbase generated from bad company 1 and their first taste of teamwork online.
Crytek have annoyed the pc fanbase by seemingly ignoring it and the console fanbase doesn't know anything about crysis to be honest, so i'd say 5m if crytek are lucky
![Smile :)](http://gamesas.com/images/smilies/icon_e_smile.gif)
Anyways, comparing crysis 2 to over marketed money spinners like halo: reach and black ops isn't fair. both of the other two are over hyped/marketed old franchises, where crysis is still a small scale newbie that's still a good game. If becoming like the two aforementioned games is what you need to get mega sales, it isn't worth it.