Each unit production structure has its own version of the Kickstarter technology pre researched. The Kickstarter technologies will allow you to train your first x Units from that type of Structure instantly
Kickstarter Technologies by Structure Level[]
there are 10 levels of each Kickstarter, tied directly to the level of the structure it belongs to. Barracks and Training Ground kickstarters instantly train 5 units per level, Stable kickstarter trains 2 units per level, and Workshop kickstarter trains 1 unit per level.
In the case of Training Grounds, the numbers are fudged a little bit, so ranks 1, 2, and 3 all provide the same bonus of training the first 15 units instantly. This is done so that players can get their very first units trained quickly while keeping the late game effects of this change minimal.
Mixed Production Structures and Kickstarter[]
You will only benefit from kickstarter technologies when you have less than your kickstarter bonus of units of the relevant type. Take a hypothetical city with 1 level 5 Training ground, 1 level 8 Stable, and 3 level 3 Barracks.
This city will be able to kickstart up to 25 Fighters or Bowmen whenever the city has less than 25 total fighters and bowmen. It will also be able to train 16 Knights or Cavalry instantly whenever the city has less than 16 total knights and cavalry. For the Barracks units, this city will be able to instantly train units whenever it has less than 45 total Swordsmen and Archers. Units from another type of structure are not relevant to benefiting from kickstarter technologies.