By “12 sections” did you mean 12 zones? The main controller can only control 8 zones. If you want to go beyond 8 zones, that’s what the expanders are for: each expander uses one additional PCA9555 to add 16 zones.
The reason the main controller only does 8 zones is that among the 16 IO pins of PCA9555, 8 are used for zone control, the other 8 are reserved for other purposes: including button 3, boost converter control on DC-powered OpenSprinkler etc.