There is a "hosts" file in which in the "db" group there are N number of hosts, you need to automatically set a variable in the playbook equal to the number of hosts in a particular group
hosts
[db] db1 ansible_host=192.168.1.101 db2 ansible_host=192.168.1.102 db3 ansible_host=192.168.1.103 db4 ansible_host=192.168.1.104 ...
example.yaml
---
- hosts: localhost
roles:
- db/example
vars:
DB_COUNT: "{{ groups['db'] | length }}"