Why does ps aux displays a number instead of a username?

Solution 1:

It means that user id is 1001 but that id has no name assigned. This can happen for example a command running in a chroot with a user defined in that chroot will not necessarily have a name assigned outside the chroot.

Solution 2:

This can happen if the username is longer than 8 characters.

Solution 3:

This is most like user's id (uid) for a user which was removed after the process was started. Or perhaps there was some kind of failure resolving username from uid.

Solution 4:

It is a uid for users whose name is unknown (e.g. an uid missing in /etc/passwd) or perhaps with strange spelling (like maybe Debian-exim, or a user with a long enough user-name, or for a bad entry in /etc/passwd)