In its opening weekend, the released Fire Emblem Fates sold over 300k across both versions, Birthright and Conquest. According to the numbers, this seems to account for the US and not the UK. While that may not seem like a massive number compared to other releases, this was in fact a major boost compared to the previous Fire Emblem title, Fire Emblem Awakening. Which, at its launch, only sold 80,000 units in the US. That title went on to be sold over 2 million times. Thus giving good future hopes for Fates.
Of the two main titles, Birthright was bought the most. Though, with the two games being vastly different in story and gameplay, this could easily swing the other way given time.
The third entry for Fates, titled Revelation, is scheduled to hit the Nintendo eShop on March 10th.