A few thousand. It's not the DRM that would have made it sell less. It's the constant FUD.

2.5-3 million

It would have sold poorly. Regardless of some opinions, people are not complete morons, and they would not freely accept the notion that their disc is useless after the first installation.
Many still equate Microsoft's DRM to digital sales, when all they wanted to do is lock out disc buyers from using those discs again. The sales would have come from hardcore XBox fans, while all rational people would refuse to be blackmailed by Microsoft, and would refuse being forcibly tracked every 24 hours.
Also, there would have been so many returned consoles from dissatisfied buyers, the press coverage would kill all sales in 2014.

Horrible idea, horrible intent by Microsoft to screw over consumers, reversing that crap policy was the best thing they've done for XOne.

5 million easily

Probably it would have sold little less in the short term, maybe it could have reached 4M anyway, thanks to early adopters, but it would have suffered heavily in the longer term.

1m or so. The bad press the X1 had before the 180 was out of this world. Had that kept going for the following months, the X1 would have sold like absolute shit (even more so than now). The reversals calmed things down a lot.

Less then a million.