If they are all linked to the same pc then a single instance of cgminer will pick them all up. However it will try to run them all at the frequency you set it to, which can be problematic if you have a unit thats not as fast as the others. But a single instance for each one means you can tweak them individually to get the best performance from each.
cool thanks gt_addict. I found this interesting post as well, where morlok8k talks about using the serial numbers as a means to configure different frequencies (I think):
https://bitcointalk.org/index.php?topic=5053833.360Zimbacard, just saw your post mentioning the same!