You need to field the indicated number of lances both in total and for the specific mission type. Normally, it's something like "3/1 Scout" or the like, where you need to assign missions to at least 3 lances, one of them with a Scout mission. The others can be set to Fight, Defend, Scout, or Training, with any other lances Unassigned. Fight and Scout missions have higher combat odds, with Defense and Training considerably lower.
Normally, you set these on Sunday, and the assignments will be used for the rest of the week's combats. The odds of each lance with a mission ending up in combat depends on the mission type, the contract difficulty and campaign difficulty settings, and the quality and current morale of the opposing force.