How much GB do I have for my iphone 5?

My capacity is 13.2GB and the available box says 51.2MB

There are 1024 megabytes (mb) in a gigabyte (gb) so about 1/20 of a gigabyte.

You have a 16GB phone, the Operating system is using about 2.5 to 3GB of it, and the rest is probably being used up by App's, Music and Photos/Videos.
Shame its not expandable with these cheap Micro SD cards like nearly every other phone.