If you want to increase or decrease resource capacity of a running overcloud, you can start more servers of a selected role or delete some servers if capacity should be decreased. To set the capacity for the compute role, first an environment file should be created:
$ cat ~/environment.yaml parameter_defaults: ComputeCount: 5
Then following command can be used to deploy it:
openstack overcloud deploy --templates [templates dir] -e ~/environment.yaml
Scaling out assumes that newly added nodes has already been registered in Ironic.
When scaling down random servers of specified role will be deleted, how to delete specific nodes is described in Deleting Overcloud Nodes.
The different scale parameters can be seen in the output of:
openstack help overcloud deploy