In practical terms, IPv6 is arranged as a 64 bit subnet ID and 64 bit device ID.

And the subnets are in a hierarchy that helps with routing but limits the packing efficiency.

We could feasibly "run out" to the point that we can't keep doing things this way.

The truth is that the address space is extremely sparse. Every ipv6 subnet is a /64 by default, yet I’m sure most of us have vastly fewer than 2^64 machines in our networks … at least for now