With the default backend (file), Glance is deployed on a single controller, because it uses a local Docker volume to store Glance images. This is explained in the documentation [1]: "By default when using file backend only one glance-api container can be running". See also the definition of glance_api_hosts in ansible/group_vars/all.yml.

If you set glance_file_datadir_volume to a non-default path, it is assumed to be on shared storage and kolla-ansible will automatically use all glance-api group members.

You can also switch to another backend such as Ceph or Swift.

[1] https://docs.openstack.org/kolla-ansible/latest/reference/shared-services/glance-guide.html