Let's say I have a PC and a smartphone, both connected to my home router.
Scenario #1. The PC and the smartphone have VPN client installed on each device. Both devices connect to the same VPN server using the same VPN protocol. The router itself is not configured to use VPN.
Scenario #2. The PC and the smartphone don't have any VPN client installed. Rather, the home router is a VPN gateway, which tunnels all traffic through the VPN.
A local passive eavesdropper (e.g., ISP), seeing outgoing and incoming traffic, tries to tell how many devices (behind the router) are currently generating traffic. Is this possible in each of the scenarioes?