How to expand the number of subuids for a linux namespace

After running something like this:

$ unshare -rUm
$ mkdir opt
$ mount --bind opt /opt
$ touch /opt/test
$ chown 1000:1000 /opt/test

I'm receiving this:

chown: changing ownership of '/opt/test': Invalid argument

As I can see, I only have 1 subuid that I can use.

$ cat /proc/self/uid_map
         0       1000          1

How can I add more than that? I already have this in my subuid/subgid files:

$ cat /etc/subuid
cloud-user:100000:65537