If you want to accurately measure movement in the human arm/hand excluding fingers and thumb, your need three tri-axial IMUs as there are 3 Degrees of Freedom (Dof) of the upper arm, 2 Dof of the forearm, and 2 Dof of the hand. Adding in the forefinger and thumb, adds 2 more IMUs.
With Parkinson's besides tremors and movement of the arm, you need to look a gait. That adds 3 more IMUs for the 7 Dof of the leg/foot. Are you going to measure both left and right? You now need 16 IMUs and you should probably add 1 more for head movement. For my own peace of mind, I would add one more on either the saddle of the hips or my preference is to add one on the beast bone and measure everything relative to it.
Even though the measurement and recording of data at 100hz is a challenge, analysis of the data may be a order of magnitude greater problem IMOSHO