On my staging server, I have
Nginx. They are on port
Given that I have to install
jenkins, on which port I have to bind it ? (which is usually 8080 but it's already taken)
Considering that Jenkins normally should not be run as root, you can take anything starting from 1024 upwards, because <1024 is reserved for root only.
I'd probably put it on port 9000, then use Nginx as a reverse proxy to map Jenkins.yourdomain.com to port 9000.
That way you'll get the bonus of being able to let Nginx do SSL too!
As @Drew correctly pointed out, you can pick any port that isn't already used.