The number of Reserves in a game will be between 10 and 12, with 10 as the initial target, and up to 2 more being added if required.
We try to ensure that each Thing in a game will be useful. So, for example, after the initial 10 Reserves have been chosen, and one of the Things in the game has Defense, we check to make sure there is another Thing with Attack also in the game. If there isn't, and there are less than 12 Reserves, a Thing with Attack will be added.
Another example is the Peng and Kun from the Chinese I expansion set. These two Things interact with each other, so if one is picked the other will be too. This may require adding an extra Reserve to accommodate.
Once the number of Reserves has been determined, the number of the base Constructs (House, Villa, Castle, Palace) in each of their Reserves is calculated.
For a 2 player game, the number of Constructs starts at 8, and for 3-4 player games, the number of Constructs starts at 12. Then, for every extra Reserve above the initial 10, 2 more of each Construct are added to the game.