A /64 will let you use up to 16 quintillion hosts on the same subnet. I'm not sure what you mean by "register" the hosts. If you're asking if you need to put them in DNS, well that's up to you

In contrast to IPv4 in IPv6 it is normal to have several IPv6 addresses configured for the same network interface. For example without configuring anything you always have at least a "link local address" within the prefix fe80::/64 where the host part (the last 64 bits) derives from the MAC address of the network interface under consideration (if it has one). This is similar to the 169.254.0.0/16 addresses in IPv4 and same in IPv6 link local IPv6 addresses only allow communiction between hosts connected to the same "link" (the IPv6 term for what is called a "subnet" in IPv4). If you have a segmented LAN, meaning several IPv6 links connected via IPv6 routers, you need at least locally routable IPv6 addresses similar to IPv4 private addresses (RFC1918). In IPv6 use the so called ULA addresses for this purpose. They stem from the prefix fd00::/8. You select another 40 bits xxxxxxxxxx by accident (see RFC4193 for theory) to form a /48: fdxx:xxxx:xxxx::/48 for use inside your LAN and from that you can form up to 2^16 /64 (fdxx:xxxx:xxxx:1::/64, fdxx:xxxx:xxxx:2::/64, ...) for addressing your LAN segments. ULA addresses are not routed through the IPv6 internet, so for communication with the IPv6 Internet you need global IPv6 addresses (up to now addresses out of the block 2000::/3 are used for that) and your IPv6-ISP must delegate a block of suitable size (/48 or /56) out of his provider space to you. As in IPv4 only global addresses need to be registered (detectable by whois), but for consumer access the block delegated to you is only visible as part of the greater ISP block registered to the whois database.