ISC DHCP server config option limit-addrs/prefs-per-ia
The limit-addrs-per-ia statement
By default, the DHCPv6 server will limit clients to one IAADDR per IA option, meaning one address. If you wish to permit clients to hang onto multiple addresses at a time, configure a larger number here.
Note that there is no present method to configure the server to forcibly configure the client with one IP address per each subnet on a shared network. This is left to future work.
There is another limit-prefs-per-ia option for prefixes. It seems a good idea even if its main/obvious use case is not supported (by ISC DHCP).