Well the other guys are correct. I do believe the FX chips that you can use with the board will only be sold in pairs.
Depending on how the board is layed out, there might be a technology issue with only running 1 cpu. In some Opteron motherboards some features, like the 2nd Gbe port, a 2nd pcie x16 slot will not be accessible with 1 cpu. Because in a dual socket Opteron board, each cpu is connected to other parts of the board through HT. If there is no 2nd cpu, there is no controller to handled the data from the other devices.