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)
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!
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.
As @Drew correctly pointed out, you can pick any port that isn't already used.