Unless stated otherwise, no more than half the total number of units in your army can be
Reserve and/or Reinforcement units, and the combined Power Ratings of all your Strategic Reserve and Reinforcement units (including those that are embarked within TRANSPORT models that are Strategic Reserve and/or Reinforcement units) must be less than half of your army’s Power Level, even if every unit in your army has an ability that would allow them to be set up elsewhere.
In these missions, Reinforcement units can never arrive on the battlefield in the first battle round (but
Strategic Reserve units can if the mission specifically states so e.g.
Recon Patrol. If during the first battle round, a unit arrives from Strategic Reserves but the mission does not instruct you where to set the unit up, use the rules for battle round 2 to determine where it can be set up). Any Strategic Reserve or Reinforcement unit that has not arrived on the battlefield by the end of the third battle round counts as having been destroyed, as do any units embarked within them (this does not apply to units that are placed into Strategic Reserves after the first battle round has started).