Detroit is no where as big a seller as Heavy Rain, but sounds like it sold more than Beyond based on digital, I would say thats disappointing, but I would say this will probably have better word of mouth/legs.
Developed by Quantic Dream, sales of the game is roughly in-line with the performance of its last title, 2013's Beyond Two Souls. When you factor in the expected increase in digital download sales (which isn't included here) it's likely the game surpassed the performance of that title - although not by a huge margin.
However, it's well behind sales of 2010's Heavy Rain, which sold almost four times as many copies (in a box) than Detroit: Become Human.
There's another new game at No.2, as well, in the form of State of Decay 2. The Xbox One exclusive was very close to Detroit, with fewer than 4,000 boxed sales separating the two.
Selling better than Beyond is good enough.