Some of the missions (like escort and prevent enemies fleeing) start with a number of enemies in relation to player team size, but then spawn enemies every turn, so a certain minimum team is needed.
That one, Prophets of the Fall, requires config 1 (large). The configs are based on the number of enemies (itself based on danger level, player team size) you will encounter. Think 9+ is needed to trigger config 1.
No, there won't be any other conditions, but each encounter will play once (with exception of escort, prevent enemies fleeing etc) over the course of a game.