So I've only played with 2 and 3, just curious what others play with? In Imperial Assault and Descent the ideal number for balance purposes is 4, so even if you have 2 people you should each control 2 heroes. Just curious if anyone here has noticed a difference in the number of players they bring. Do more monsters spawn? Do you have less time with more? Is there an 'ideal' number for MOM?
I always play with 4; more options, and a better control of the board. Still, 3 possibly it's the best number to go: sometimes with 4 you're able to have some "spare actions" in the last quarter of the game, so, the tension could drop
I've played with all player counts and first of all i would say it depends on the scenario.
Second i actually think its one of those cases where it "nearly" works with all player counts, i dont particularly like it with 2 players. I love it with 3, 4 and 5. And to talk about the balancing of the missions, how they are balanced depending on number of players, depends on the scenario.
In all scenarios, monsters will have more health if you are more players simple as that.
In some scenarios at least, tests have been harder if we have had more players.
Finally the time to get on with the investigation is shortened by the number of players, this counts for especially scenario 1, 4, 5 and 6. Meanwhile scenario 3 throws the "monster attack" more often with 5 players than with 2 for example
And with 5 players theres that added chance that someone will become a traitor 
I've noticed that four investigators seem to be ideal. Three is too few if you want to win as there are too few actions and fumbling an attack etc. really eats up actions. And since the players are often in a hurry to reach a goal, you cannot spend too much time investigating what seem like interesting but not crucial objects or not talk to NPC etc.. And that leaves out bits of the story.
Mind you, these are my thoughts and experiences. And while I claim that 4 investigators seems ideal, I've yet to play the game with four investigators. I've played the game with 3 and 5 investigators. Multiple games with both player amounts. With 5 players, there can be some boring moments and with 3 players, there seems to be too few actions.
Edited by Elrath