Let's say you want to use Ansible to do something against a large number of managed nodes (e.g. target systems), such as 100 managed nodes. By default, Ansible will create 5 forks, which means that Ansible will perform a task against 5 managed nodes, then the next 5 managed nodes, and so on, until the tasks has been completed against all of the managed nodes. In Ansible terminology, this is known as parallelism.
Command line option -f or --forks can be used to specify the number of fork, like this.
ansible-playbook playbook.yml --forks 25
Or, you could define the default number of forks in ansible.cfg.
forks = 25