I can be wrong, but I think devs have confirmed several times that the duration setting of the mission is not really implemented, so you can put it to whatever you want, it won't affect your missions.
The number of enemies is random. You can get standar missions with 10 enemies in a phase. Remember to put the mission to hard, so enemies are 1 level higher than you, which will get much better XP rewards and feeling of toughness.
