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")}}