My hosting company provides a /64 IP6 subnet. What is best practice when assigning IP6 addresses. Should each site have its own dedicated IP even though they share an IP4 address - or should they also share an IP6 address?
It doesn't matter. Every IP address (v4 or v6) registered in Plesk must be designated as either dedicated or shared.
Actually, it's up to you which IP address you will set up as dedicated or shared.