You'll want more than one, at least a pair. They're a lot of fun--look like little helmets running around, but really noisy.
They do tend to roam, so unless you have understanding neighbors (I didn't!) you'll need to keep them penned.
I sold all of mine this fall because I thought that I would be moving by the end of the year and was trying to downsize. However, I haven't found a place yet, but plan to get some more once I'm relocated.
Before I had to pen the guineas my blind cattle dog would herd them; she could hear them and it gave her something to do. I hated to take that away from her.