Storage - IdleOn MMO Wiki
There are 3 different types of Storage available; Inventory Bags, Capacity Pouches and Storage Chests. Inventory Bags increase how many inventory slots a character has, Capacity Pouches increase how many items a character can hold per stack, and Storage Chests increase the number of slots in your account...
Unique buildings | Stellaris Wiki
With the max building slots of 6, only six building can be built in total.
Universal Way to Unlock New Building Slots?
Though there is no cheat to unlock more Building Slots in HOI4, you can use a combination of cheats to get what you want.