Well, if you have multiple layer 2 networks, you need multiple subnets, and you should assign a /64 per layer 2 network for stateless autoconfiguration to work,

Well, if you have multiple layer 2 networks, you need multiple subnets, and you should assign a /64 per layer 2 network for stateless autoconfiguration to work, so splitting a /64 into smaller pieces is not a good idea, therefore an ISP should give you at least a /56, better a /48, that you can subnet for any layer 2 networks that you might have.