We don't know, that's one of the infuriating things about steam is that valve will not release any of the actual sales figures.
The top sellers lists they publish is probably daily
There was a big stink about this and I seem to remember that the evidence amounted to nothing more than an idea for a zombie survival game called "WarZ" nothing about the form of the game.
It's obviously has had some development time, but how much and what was done when is questionable.
WarZ does have better animations over DayZ, esp the zombies, but it's had major issues with hackers and is basically unfinished, which is a total mistake when you have the gall to to charge people not just for the game but also micro transactions, so you can end up paying real money for items which you'll lose when you're killed.
One of the ideas they where talking about that I really liked was that it would have a player created mission system, a way players could create basic MMO type missions, a feature that is not implemented
AFAIK