As of this writing, the latest openstack-base bundle openstack base | Juju, with the raw yaml of the bundle being https://api.jujucharms.com/charmstore/v5/bundle/openstack-base-70/archive/bundle.yaml
Starting on line 13 of the bundle, we define three machines:
All the individual OpenStack applications are then deploy directly to those machines, or to lxd containers on those machines with
to directives. For example, starting on line 220, you can see that keystone is deployed to a lxd container on machine 0:
The short answer is that you’re only getting three nodes because the openstack-base bundle only deploys to three nodes. To expand to more nodes, you’d need to do some thinking about which applications you want to deploy where, and edit the bundle accordingly – e.g., add more machines to the machines list, then update the -to directives to place the machines.