diff --git a/roles/matrix-synapse/defaults/main.yml b/roles/matrix-synapse/defaults/main.yml
index 058d4a3c..31af47bc 100644
--- a/roles/matrix-synapse/defaults/main.yml
+++ b/roles/matrix-synapse/defaults/main.yml
@@ -320,6 +320,9 @@ matrix_synapse_workers_pusher_workers_metrics_range_start: 19200
 matrix_synapse_workers_appservice_workers_count: 1
 matrix_synapse_workers_appservice_workers_metrics_range_start: 19300
 
+# matrix_synapse_workers_federation_sender_workers_count can only be 0 or 1 for now.
+# More instances are not supported due to a playbook limitation having to do with keeping `federation_sender_instances` in `homeserver.yaml` updated.
+# See https://github.com/matrix-org/synapse/blob/master/docs/workers.md#synapseappfederation_sender
 matrix_synapse_workers_federation_sender_workers_count: 1
 matrix_synapse_workers_federation_sender_workers_metrics_range_start: 19400
 
diff --git a/roles/matrix-synapse/tasks/validate_config.yml b/roles/matrix-synapse/tasks/validate_config.yml
index d05718d9..283483b4 100644
--- a/roles/matrix-synapse/tasks/validate_config.yml
+++ b/roles/matrix-synapse/tasks/validate_config.yml
@@ -20,6 +20,7 @@
   with_items:
     - "matrix_synapse_workers_appservice_workers_count"
     - "matrix_synapse_workers_pusher_workers_count"
+    - "matrix_synapse_workers_federation_sender_workers_count"
 
 - name: (Deprecation) Catch and report renamed settings
   fail: