Unifi Network: Number of Active Clients on Guest SSID

I actually ended up making a template sensor for this rather than relying on the template in a dashboard card:

template:
  sensor:
      - name: "guest.ssid Device Count"
        state: "{{ states.device_tracker | selectattr('attributes.essid','defined') | selectattr('attributes.essid','eq','guest.ssid') | list | count }}"
        state_class: measurement
        unique_id: "guest_ssid_device_count"
        icon: "mdi:devices"

Then read that in the template:

 {% set dev_count = states('sensor.guest_ssid_device_count') | string %}{{iif(dev_count=="1","1 guest device connected",dev_count+" guest devices connected")}}
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