I am planning to try this out, once my Porcupine boards plus my second Parallella arrive!
Read the architecture reference http://www.adapteva.com/docs/epiphany_arch_ref.pdf and see that the maximum size grid is 32 x 32 cores or 8 x 8 Epiphany chips.
Now to get the inter board elink working well you are likely to want to get data on demand from the epiphany chip to the CPU. At the moment that is just a polled interface so the Arm cores can poll a message buffer over elink into the Epiphany chip. But soon, if not already, you should be able to use https://github.com/parallella/oh/tree/master/emailbox that will interrupt the CPU from the Epiphany side.
Also because the Parallella CPU uses one of the four elink connections and the Parallella board does not have a connection to the second elink connection (http://www.parallella.org/docs/parallella_schematic.pdf) you will be limited to 8 Parallella boards in a row... but wait there is an FPGA on the board with a lot of spare gpio pins... so it is not beyond the wit of man to connect the CPU elink connection to a home grown elink like connection (perhaps with for example 4 bit width rather than 8 bit width) from the FPGA so with a bit of HDL work you might be able to get 16 Parallella boards connected up...
You are also likely to need to do a bit of Linux kernel driver work to get the whole thing up and running.