I'd say 500k MAXIMUM. Such an old game. Full price (here in Canada at least). I will be picking it up one day, but I doubt this will reach close to 1 million. Arkham City port on Wii U was only a year old, but with enhanced features and launched with the system, and according to this site couldn't even reach half a million.
Skyrim is far more popular than Batman: Arkham City. Skyrim on the Xbox 360 has almost just as many sales as Batman: Arkham City's combined lifetime sales on every platform. Also, I would argue that comparing sales of a game that was ported to the Wii U as the basis of what a game would sell on the Switch, would be an incredibly inaccurate depiction of Switch sales potential.
Games and debauchery.