The most trustful sources Codex and reddit.
Other than that? I know it's Codex's wishful thinking for PoE2 to flop, because of Avellone drama, but are there actually any numbers behind that claim? I am really curious, because I like both games, and I would not be very happy, if the IP would go six feet under...
Steam API leak showed 200k copied with any achievement.
Which is pretty disappointing but game pricing is not indicating a major flop.
Just under-performance to expectations.