Sadly sales stats for prior games aren't easily available, but it's almost certainly one of the best selling games in the series.
FromSoftware has announced that Armored Core 6: Fires of Rubicon, has surpassed 3 million units sold, a fantastic milestone for a franchise that previously firmly existed in the ocean of niche video game franchises. The game is just under one year old by the time of this announcement, which certainly isn't bad pace for a new entry in a legacy IP.
As for how this compares with previous games in the franchise, these stats are somewhat hard to track. VGChartz is the only website with figures for the majority of the franchise, and its stats show prior entrees have never breached the one million sales milestone. The closest was Armored Core 2 on the PS2, which apparently shipped roughly 790,000 copes in its lifetime. With that in mind, you can see how wild the sales for Armored Core 6 actually are.
It's worth noting that back in October 2023, the Japanese voice actor for Rusty announced the game had passed 2.8 million units sold, so the momentum has clearly slowed in the game's later months post-release. But even still, these sorts of games with a strong single player campaign and plenty of replayability have proven consistant sellers even years after launch, so one can expect this figure to slowly creep up in the coming months. I wouldn't expect it to pass 4 million, mind you.
This begs the question, why has Armored Core 6 sold so well compared to its precessecors? There are some obvious factors, such as FromSoftware's vastly increased popularity following the release of the Dark Souls series and Elden Ring. The popularity of those titles will have surely bled over, especially as Armored Core 6 has inhereted some of the features present in said games. But, it's worth noting that while niche, Armored Core is a well-respected series in the minds of the gaming community. Good will, fond memories, and a solid track record can do a lot in this day and age.
If you've not picked up Armored Core 6, let us know why below, as well as whether you reckon we'll get another one in the coming years as a result of these sales figures!