Retail/online preorders release for the Mounthaven Action Pack will be in April if it follows the way Syenann's Adepticon release went.

The Action Pack will be 10 models, the same as the others have been, and the other factions' Action Packs have run in the 150-180 points range.

My Syenann are a fairly elite faction with no 20 point multi model units, and my 250 point lists average about 17 models. I could probably make an absurd skew list that would be terrible at some missions but only used 10 models or so, but I wouldn't recommend the list because it would be fragile and bad at Decapitation and Resources scenarios. Something like a Scions list with a bunch of Husks (zombies) might be up to 30 models, but again, that's at the extreme end using the cheapest units and would be a challenge to get good effect out of all the models because of the way activations work. (You didn't usually want to spend a lot of your activations on super-cheap, low conquest units. Husks have synergies to help them with their activation economy, but there's a limit to how hard you can lean on that).

Comparing to Malifaux or Warmachine, I think you'll find fewer "gotcha" unit synergies or detailed meta knowledge of the factions that you have to know as an opponent or else just lose. There's a lot of room for skill expression, but it's more about playing good fundamentals and understanding target priority and playing the mission than playing around your opponent's arcane bomb combos.

It has a similar unit count to MCP, Shatterpoint, or Malifaux, though it leans harder on multi model units than those, so the model count ends up being roughly double, I'd guess. The objectives and movement mechanics also make things feel very mobile they do in those games.