I am currently thinking about upgrading my 4 camera system to a 6-8 cameras for two people tracking. In my case, plugging 6 cameras in my current computer is easy. But for 8 cameras, I have to change a computer then upgrade it which is a spot of bother. Now I am wondering to ask how much difference between a 6 and 8 cameras in terms of tracking accuracy? Or possibly something other aspects?

Basically, 6 cameras are pretty enough for capturing a single actor. Using 8 cameras makes sense when capturing 2 actors to decrease the mutual occlusion of the actors.

thanks very very much!!!

I have a question about frame drops. How could I justify the frame drop rate as an acceptable rate. Is there any statistics for that?

For example, when I link two ps eye cameras on my usb 3.0 controller, it drops 30-35 frames + 30-35 bad frames in a five minutes recording.
However, if I am using usb 2.0 controller to do the same thing, it will only have 3-4 frame dropped and 3-4 bad frames.

There is no statistics on the influence of bad/dropped frames on the tracking process.
As a rule of thumb, I would take that more than 1% of dropped/bad frames is totally unacceptable, and leads to lots of tracking errors. Acceptability of lower values may depend on the speed of motions (slow motions are more tolerant to frame drops), the number of cameras (more cameras - more tolerance).
Considering the above, your values seem OK.