As long as you are connected you will have an encrypted tunnel into your entire "home" network as if you are connected locally - so you could typically see everything on your network, depending on your common access rights on other devices/computers on your network.
If a single RPi is enough or not depends on
- others tasks devoted to that RPi
- performance requirements you might have for various needs
- need to support multiple users connected simultaneously
If the RPi just runs NR with a "normal" flow with low cpu load and you just want access to the Dashboard for gui reasons, I think a single RPi will be able to handle that fine
Myself, I decided to install the VPN server on a separate RPi3. Reasons for this are that I wanted to have the possibility to override regional restrictions while streaming live TV channels from abroad. The streaming works very well when a hotel provides a good network connection. But for normal Dashboard operation, I think a RPi3 dedicated for the VPN server is a bit overkill