Window groups and counters

In groups.yaml

Create a group of windows per room:

master_bedroom_windows:
  name: Master Bedroom Windows
  entities:
    - binary_sensor.master_bedroom_left_window_state
    - binary_sensor.master_bedroom_right_window_state
    - binary_sensor.master_bedroom_bathroom_window_state
  icon: mdi:window-closed-variant

..and a group of groups..

upstairs_windows:
  name: Upstairs Windows
  entities:
    - group.master_bedroom_windows
    - group.large_bedroom_windows
    - group.small_bedroom_windows
    - binary_sensor.office_window_state
    - binary_sensor.upstairs_bathroom_window_state
  icon: mdi:window-closed-variant

in configuration.yaml

add a sensor that tracks the count of active entities

- platform: template
  sensors:
    open_windows_upstairs:
      value_template: >
        {{ expand('group.upstairs_windows') | selectattr('state', 'eq', 'on') | list | count }}
      icon_template: 
        mdi:window-closed-variant