elpa-linum-relative software package provides display relative line number in Emacs, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install elpa-linum-relative
elpa-linum-relative is installed in your system.
Make ensure the elpa-linum-relative package were installed using the commands given below,
$ sudo dpkg-query -l | grep elpa-linum-relative *
You will get with elpa-linum-relative package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Lisp |
| Package | elpa-linum-relative |
| Version | 0.4-1 |
| Description | display relative line number in Emacs |
| Sub Section | Lisp |