diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index 1d32760a7bcf..2409c6f5ba7f 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -267,6 +267,10 @@ async def action_for_event_by_user(
 
         room_members = await self.store.get_joined_users_from_context(event, context)
 
+        room_member_count = await self.store.get_number_joined_users_in_room(
+            event.room_id
+        )
+
         (
             power_levels,
             sender_power_level,
@@ -278,7 +282,7 @@ async def action_for_event_by_user(
 
         evaluator = PushRuleEvaluatorForEvent(
             event,
-            len(room_members),
+            room_member_count,
             sender_power_level,
             power_levels,
             relations,
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index abfc56b0616c..177b9a90c5aa 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -75,6 +75,9 @@ def _invalidate_state_caches(
             self._attempt_to_invalidate_cache(
                 "get_users_in_room_with_profiles", (room_id,)
             )
+            self._attempt_to_invalidate_cache(
+                "get_number_joined_users_in_room.invalidate", (room_id,)
+            )
 
         # Purge other caches based on room state.
         self._attempt_to_invalidate_cache("get_room_summary", (room_id,))
diff --git a/synapse/storage/databases/main/cache.py b/synapse/storage/databases/main/cache.py
index a07d48f66c95..17f7bf66bf1b 100644
--- a/synapse/storage/databases/main/cache.py
+++ b/synapse/storage/databases/main/cache.py
@@ -218,6 +218,7 @@ def _invalidate_caches_for_event(
             self._membership_stream_cache.entity_has_changed(state_key, stream_ordering)
             self.get_invited_rooms_for_local_user.invalidate((state_key,))
             self.get_local_users_in_room.invalidate((room_id,))
+            self.get_number_joined_users_in_room((room_id,))
 
         if relates_to:
             self.get_relations_for_event.invalidate((relates_to,))
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 46a004ce7985..5787f4fc559d 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1770,6 +1770,10 @@ def _store_room_members_txn(
                 self.store.get_local_users_in_room.invalidate,
                 (event.room_id,),
             )
+            txn.call_after(
+                self.store.get_number_joined_users_in_room.invalidate,
+                (event.room_id,),
+            )
 
             # The `_get_membership_from_event_id` is immutable, except for the
             # case where we look up an event *before* persisting it.
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index ac6f568e6852..9bb83ac8ac75 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -337,6 +337,15 @@ def _get_room_summary_txn(
             "get_room_summary", _get_room_summary_txn
         )
 
+    @cached()
+    async def get_number_joined_users_in_room(self, room_id: str) -> int:
+        return await self.db_pool.simple_select_one_onecol(
+            table="current_state_events",
+            keyvalues={"room_id": room_id, "membership": Membership.JOIN},
+            retcol="COUNT(*)",
+            desc="get_number_joined_users_in_room",
+        )
+
     @cached()
     async def get_invited_rooms_for_local_user(
         self, user_id: str