Ansible Example of Multple Play in One Playbook and Multple Playbook in One Yaml files

Example of Multple Ansible Play in One Playbook

---
- name: sample1
  hosts: localhost
  gather_facts: no
  tasks:
    - name: sample_task1
      debug:
        msg: "sample 1 task 1"

- name: sample2
  hosts: localhost
  gather_facts: no
  tasks:
    - name: sample_task2
      debug:
        msg: "sample 2 task 2"

Example of Multple Ansible Playbook in One Yaml file

Build many sub-playbooks and aggregate them via include statements.
- include: playbook-one.yml
- include: playbook-two.yml

Note - 
include is deprecated. docs.ansible.com/ansible/latest/playbooks_reuse.html import_playbook: foo is the right way to go

For newer versions of Ansilbe, you can build many sub-playbooks and aggregate them via import_playbook statements:
---
- import_playbook: A-systemd-networkd.yml
- import_playbook: B-fail2ban-ssh.yml
- import_playbook: C-enable-watchdog.yml
Rajesh Kumar