A /64 is a 64-bit network mask on the 128bit ipv6 address, effectively leaving you with 64 bits for host addresses on that subnet. A /128 is a 128bit mask, leaving you with 0 extra bits (ie: a single host address).
2^(128-n) is the way it works for HOST addresses (no '-1', as in ipv4, as there is no broadcast address in ipv6, and 128-128=0, so 2^0=1, a single host address). 2^n is the way it works for networks/subnets, so a /128 leaves 2^128 possible networks, each with 2^0 hosts on them, while a /127 leaves 2^127 networks, each with 2^1 hosts on them.
A /64 is equal to the square of the entire existent ipv4 space (/32), and should be more than enough for any subnet anywhere, ever (it's roughly 18,446,744,073,709,551,615 IPs).
The issue is that many of the 'fundamental' protocols are built on assumptions that the smallest network will be a /64, and if your ISP gives you 1 /64, you can't have more than one 'network' on your end of the connection - even if you can have 18 quintillion addresses, they all have to be on the same subnet, so no fire-walling/etc. within your network.
