Without knowing the concurrent users on other platforms, having 66k peak means nothing in extrapolating how much it sold on PC vs the other platforms.
Also back in Odyssey release
Ubisoft big PC success was driven mainly by Anno 1800 and Uplay.
www.pcgamer.com
PC was leading platform, just not only steam
Steam Spy automatically gathers data from Steam user profiles, analyzes it and presents in simple, yet beautiful, manner. Steam Spy is designed to be helpful for indie developers, journalists, students and all parties interested in PC gaming and its current state of affairs.
steamspy.com
Estimated 2-5M units on Steam. SteamDB tracks it at 3.7-5.47M owners.