How many terminal bars N / PE do I really need?
First of all a warning. If you are not sure with working on mains then stay away from it. Furthermore for safety always remove the power from the incomming line by opening the corresponding breaker.

If you look good then you can see that the connective bar is separated into 2 parts. One part for (P)rotective (E)earth (yellow/green) and the other part for (N)eutral (blue). 

You can use a insulated bar any way you like. In this kind of connection boxes a non insulated bar should only be used for PE(green/yellow) connections.