I have a pretty good ISP - I get a /56 but the original idea was /48 for everyone. I only have 256 subnets available. With /48 I would have say 256 x 256 subnets which would be ideal for my family. With /56 I have very little elbow room.
Each subnet (/64) is 18,446,744,073,709,551,616 addresses - which is nice.
/56 == 256 /64s Each /64 == 18,446,744,073,709,551,616 addresses