It was in the top sellers about as long as FC was, actually. It also got higher into them than FC did, all data considered. Both of them stayed really high on GOG for significantly longer than on Steam, but well, that's GOG. It isn't as indicative as it might be; it mostly means that there were way more purchases in a huge wave at release and that petered out. It doesn't really show in a useful way how many sales those actually were; Steam's best seller list is a confusing mess, and even a bit buggy at times.
That said, more than that, just... don't assume. At this point, all of the assumptions a lot of people are starting to make, are getting just plain toxic, and we have a lot more time to gain more sales, especially considering that SC's release got FC a lot of attention, including new purchases. I for one feel quite optimistic. SC's launch has also gone much smoother than FC's overall, with the bugs being much less gamebreaking or widespread. God am I ever thankful for that...