I need to execute a software (unison) and see at the end of the
process how much data it has sent and received.
How can I easily do this?

Trickle is a great software to limit the bandwidth, that can be used
in user-mode:
For instance, to limit the download bandwidth consumption of unison to
50 Kb/s:
$ trickle -d 50 unison backup

In a similar way, is there a simple tool that tells me how much data
has bent sent and received by an application?
something like this:
$ datasocketcount unison backup
Data Socket Count report:
Data sent: 2340032 bytes
Data received: 54030 bytes

Many thanks,