I just want to point out, that it is possible to set up a serial communication with more than two Arduinos. I2C is definitely much better, but here is what you would have to do. You have to setup a ring. TX 1 => RX 2, TX2 => RX 3, TX3 => RX 4, .... TX n => RX 1. Each Arduino needs to know its ID. Each send message needs to send the ID of the receiver. When an Arduino receives a message it compares its ID with the message ID. When equal you react to the message (consume), if not you send it to the next node. Not very efficient, but definitely possible.
Serial communication with more than two Arduinos
I just want to point out, that it is possible to set up a serial communication with more than two Arduinos. I2C is definitely much better, but here is what you would have to do. You have to setup a ring. TX 1 => RX 2, TX2 => RX 3, TX3 => RX 4, .... TX n => RX 1. Each Arduino needs to know its ID. Each send message needs to send the ID of the receiver. When an Arduino receives a message it compares its ID with the message ID. When equal you react to the message (consume), if not you send it to the next node. Not very efficient, but definitely possible.