ansible/examples/playbooks/batch_size_control.yml

# ordinarily, without the 'serial' keyword set, ansible will control all of your machines in a play at once, in parallel.
# if you want to perform a rolling update, so that each play completes all the way through on a certain number of hosts
# before moving on to the remaining hosts, use the 'serial' keyword like so:

---
- hosts: all
  serial: 3

# now each of the tasks below will complete on 3 hosts before moving on to the next 3, regardless of how many
# hosts are selected by the "hosts:" line

  tasks:

  - name: ping
    action: ping
  - name: ping2
    action: ping