Its safe to assume a 24m install base ending 2016, and that follows from assuming 2016>=2015>2014-launch. Moreover, suppose we are looking at a shorter gen then sales won't peak higher or much higher from that. Perhaps 40m LT seems like a good upper bound.
As far as if those are good sales, it depends on how it overall turns out for M$ and all other software companies involved in the long run. If M$ sees significant profits from XOne hardware and software, same for third party software, then it's strong to reject the idea it sold poorly. The GameCube may have sold little but Nintendo didn't suffer from it. Finally, total sales of a console can affect the brand, and this in turn may affect the next generation either positively or negatively.
e=mc^2
Gaming on: PS4 Pro, Switch, SNES Mini, Wii U, PC (i5-7400, GTX 1060)