What kind of topology do you want to make?
- a pipeline of Arduino's? (A-> B -> C -> D etc)
- a tree? (A -> B, C; B -> D,E; C -> F,G etc
- a meshnetwork (all to all)
- a 'square' mesh: (Xn -> Xn+1, Yn)
- a thee layer neural network
What is the datarate needed?
If you want multiple computers to communicate use one of the communicating shields zigbee, ethernet, wifi or so.
But there are other ways to communicate, you can create sort of parallel bus or using blinking leds and photosensors in combination with Morse. Or servo's with flags and a distance sensor