Hello!
IPv6 addresses are divided into /64 subnets. That seems to be the most common ownership boundary. However, it hasn’t mattered much so far because most working satellites are currently hosted on the Google Cloud Platform, which (still, in 2020, I know!) doesn’t support any IPv6 traffic.
As to nodes with multiple addresses, we currently sidestep the issue by having each node advertise a single preferred address. That address can change as long as the node keeps the satellite updated (assuming the node is actually contactable at that address). IP subnet filtering, I believe, applies to the last IP that the satellite resolved for a given node’s advertised address.