Hello @VariableEmployee,
I2C is not designed to run on long lines - so theoretically you can connect 30 devices if the addresses do not collide but it is not a good idea to do it like this due to the long wires.
Also, there is no direct access to the I2C on Neurons, it is available only on UniPi 1.
If you want to use it as a PLC, meant as a robust and reliable automation controller, I suggest connecting the peripherals via RS485...
UART is also a low-level interface (such as I2C), in installation cabinets, these kinds of protocols is used only for the communication within one device. Protocols like RS485, RS232 and others are used to communicate with other devices.