After mission 5 "The true Enemy" the bar below the different ranks starts to have one or two lines with symbols, these are the lines the total mission score need to reach above for you to get either one or two extra deployments that day.
You get it by simply reach a good rank fast. Good missions for this is defense mission or mission where you don't have to stop along the way to secure resources. You should also pick up each of the automated foundries during your missions to get a bonus toward achieving these. The more you have the easier it will be to obtain extra deployment. but don't worry you have plenty of missions to achieve this on.
One such mission where you can easily get this is Mission 8 "The Defense of Argus Gate" if you missed your chances on your first defense mission and wanted to secure a resource on the other.