Probably between 200 to 500k same range as the rest of the NBA/NHL playoffs games thus far
233k avg of avg viewership for NHL on truTV simulcasts.
Ranging from a low of 81k to a high of 462k.
Those were all true simulcasts though.
Alt-Casts on truTV in the regular season:
Chiclets Cast did 42k.
MultiVersus did 28k.
Going off the numbers it shows a difference between simulcast, people just wanting to watch the traditional broadcast of the game and they end up on truTV (tracking it I've noticed a lot of people watch on truTV and don't even realize they're not on TNT while thinking they are on TNT) and alt-casts where it's a different broadcast of the game.
So I'd project on the lower end. With a lot of complaints early in the game from the people who need their hands held with things like: "why isn't TNT doing a real broadcast?! What is this bs. I want to watch the regular broadcast! TNT sucks!" with a lot of red face emojis. See it every time from a surprising amount of people who are incapable of figuring out what channel they're actually on or using any amount of common sense to figure out that 'hey, this looks like an alt-cast, I should probably find the normal broadcast of the game is that's what I want to watch [instead of running to X/twitter/forum/etc. to complain that they aren't showing a regular broadcast of the game as if it's not an option]'
That said, simul or alt, "casts" always boost #'s so it'll add to the # in whatever way it does. I'd be surprised though if it was closer to the simulcast playoff high than the simulcast playoff low.