IPv6 is not the same as IPv4. In IPv6 you assign a short prefix to each end-user site, so they are able to have as many subnets (/64s) as they need/64s) as they need. You should not be concerned with exhausting the IPv6 addressing space, and you should think big when planning future requirements. If you need more space, you can go back to your RIR and, providing your addressing plan justifies it, you can obtain more IPv6 addresses.