here's some info on how it is broken down:

Each /48 IPv6 prefix allows a site (a university, an organization, a company, ...)
to have up to 2^16 subnets (2^16= 65.535 subnets).

Each subnet could handle 2^64 nodes ( 2^64 = 18.446.744.073.709.551.616 nodes -
each with distinct IPv6 address).

Therefore, if you calculate the total number of unicast IPv6 addresses you get
with a single /48 prefix, you get:

2^16 subnets x 2^64 nodes = 1.208.925.819.614.629.174.706.176 IPv6 addresses.