There are a few different terms we use in WC in regards to Special Forces that we've done a bad job of explaining thus far. Here's what they mean and how they relate to prize earning:
Build Limit: This is the total amount of units of a certain type that you can CURRENTLY build. For example, if your current build limit for Operators is 5, you can only build 5 Operators. This doesn't mean that you can't earn the ability to build more, but only that you are currently capable of building 5.
Limit Increase: Purchasing or earning a Limit Increase increments your Build Limit. For example, purchasing a Limit Increase from the Gear Store means that you now have the right to build an additional unit of that type, as it raises your Build Limit by 1. Most prizes or store purchases are Limit Increases.
The prizes in Onslaught are Limit Increases. Let's say you currently have 5/5 Operators in your Barracks right now. Playing Onslaught and completing a wave that grants you one Operator will increase your Build Limit for Operators to 6, making it so that your Operator count in the Barracks will appear as 5/6.
Unlock: Fairly self-explanatory save for one small caveat: unlocking a unit (say, from the Gear Store) not only allows that unit to now appear in its respective production building (Barracks for infantry, War Factory for vehicles), but also grants you a Limit Increase at the same time. So where before an unlock you had 0/0, after an unlock you have 0/1 for the unlocked unit.
Max Purchasable: This is the maximum amount of Limit Increases for a unit you can buy from a store. This is not the same as Max Buildable, which is explained below.
Max Buildable: Every Special Forces unit has a maximum Build Limit that players cannot progress beyond until we (KIXEYE) raise the Max Buildable. This is referred to as Max Buildable. The Max Buildables for each unit are as follows:
Heavy Operator: 12
Lead Operator: 12
Lead Heavy Operator: 12
Lead Preserver: 16
Lead Infiltrator: 9
Brother Jeremiah: 1
IMPORTANT NOTE: You cannot receive a Limit Increase for a unit that you are currently at the Max Buildable for. So, if you complete an Onslaught wave that grants a Limit Increase for Operators and you are currently at 48 Operators, you will NOT receive that prize. If you own all prizes in a given prize set, some activities (i.e. - Shadow Ops) will grant you a fallback prizes. NOT ALL ACTIVITIES SUPPORT FALLBACK PRIZES YET. We're working on implementing fallback functionality for all activities in a future release.