Welcome aboard! :)
I started out a couple of years ago using three Logitech web cameras; with four, I saw a big improvement in accuracy. Since that time I've upgraded to four PS3 cameras, and I recently added two more cameras to my system but I haven't tried the six camera setup yet.
IMO, a minimum of four is recommended but three may work if you limit the type of actions you perform. For example, with only three cameras you may have trouble tracking a performer walking around because the cameras cannot see enough of the body to track it accurately. With less spatial data, you may see a bit more jittering too. But if you're just testing the software, three cameras might give you a good idea of what's possible.
I have a couple of test videos posted on my YouTube channel showing what four cameras can do:
LGD TestTubeBear in mind that the sword test was done using an older version of iPi Studio that didn't have shoulder tracking. Even so, it's not bad.
The 'stretch and walk' video was done just a couple of months ago with a version of iPi DMC that had an excessive jitter problem. Again, even so, the mocap is pretty decent for raw, unedited mocap. IMO, this mocap looks a little linear and 'robotic', which I believe is the result of over-aggressive Jitter Removal, another problem with that build. BTW, this version of iPi DMC did have shoulder tracking, which is very apparent in some of the motions. (This performance was designed to test character deformations in my short film but, as it turns out, it was also a good test of some of the mocap system's limits.)
I'm happy to say that
the latest version of iPi Studio is much improved: the 'excessive jitter' bug is gone and the new Configurable Jitter Removal is a powerful feature. Yesterday I re-tracked the four camera 'stretch and walk' test using the new version, and the mocap looks significantly more stable and accurate than what you see in these older test videos. I'll try to post a comparison video in the next day or two. Additionally, I hope to have new test videos using the six-camera setup posted on the channel this upcoming week.
G.