Consider to stop instances that are no longer performing tasks, so as not to run them unnecessarily.
$ openstack server stop <instance>
Please note that if you have associated a floating IP with your VM and it contains a domain name, you must dissociate your floating IP before deleting it. Otherwise, you will lose your DNS declaration and will need to create a new floating IP by specifying your domain name.
Consider removing useless instances.
$ openstack server delete <instance>
You will free up resources to create new instances.
Furthermore, it’s an eco-responsible approach !