diff --git a/swh/storage/proxies/buffer.py b/swh/storage/proxies/buffer.py
--- a/swh/storage/proxies/buffer.py
+++ b/swh/storage/proxies/buffer.py
@@ -9,7 +9,7 @@
 from typing_extensions import Literal
 
 from swh.core.utils import grouper
-from swh.model.model import BaseModel, Content, SkippedContent
+from swh.model.model import BaseModel, Content, Directory, SkippedContent
 from swh.storage import get_storage
 from swh.storage.interface import StorageInterface
 
@@ -37,6 +37,7 @@
     "content_bytes": 100 * 1024 * 1024,
     "skipped_content": 10000,
     "directory": 25000,
+    "directory_entries": 200000,
     "revision": 100000,
     "release": 100000,
     "snapshot": 25000,
@@ -65,6 +66,7 @@
               content_bytes: 100000000
               skipped_content: 10000
               directory: 5000
+              directory_entries: 100000
               revision: 1000
               release: 10000
               snapshot: 5000
@@ -80,6 +82,7 @@
             k: {} for k in OBJECT_TYPES
         }
         self._contents_size: int = 0
+        self._directory_entries: int = 0
 
     def __getattr__(self, key: str):
         if key.endswith("_add"):
@@ -105,7 +108,8 @@
             object_type="content",
             keys=["sha1", "sha1_git", "sha256", "blake2s256"],
         )
-        if not stats:  # We did not flush already
+        if not stats:
+            # We did not flush based on number of objects; check total size
             self._contents_size += sum(c.length for c in contents)
             if self._contents_size >= self._buffer_thresholds["content_bytes"]:
                 return self.flush(["content"])
@@ -119,6 +123,17 @@
             keys=["sha1", "sha1_git", "sha256", "blake2s256"],
         )
 
+    def directory_add(self, directories: Sequence[Directory]) -> Dict[str, int]:
+        stats = self.object_add(directories, object_type="directory", keys=["id"])
+
+        if not stats:
+            # We did not flush based on number of objects; check the number of entries
+            self._directory_entries += sum(len(d.entries) for d in directories)
+            if self._directory_entries >= self._buffer_thresholds["directory_entries"]:
+                return self.flush(["content", "directory"])
+
+        return stats
+
     def object_add(
         self,
         objects: Sequence[BaseModel],
@@ -179,5 +194,7 @@
             buffer_.clear()
             if object_type == "content":
                 self._contents_size = 0
+            elif object_type == "directory":
+                self._directory_entries = 0
 
         self.storage.clear_buffers(object_types)
diff --git a/swh/storage/tests/test_buffer.py b/swh/storage/tests/test_buffer.py
--- a/swh/storage/tests/test_buffer.py
+++ b/swh/storage/tests/test_buffer.py
@@ -3,6 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+from collections import Counter
 from typing import Optional
 from unittest.mock import Mock
 
@@ -280,6 +281,29 @@
     assert s == {}
 
 
+def test_buffering_proxy_storage_directory_entries_threshold(sample_data) -> None:
+    directories = sample_data.directories
+    n_entries = sum(len(d.entries) for d in directories)
+    threshold = sum(len(d.entries) for d in directories[:-2])
+
+    # ensure the threshold is in the middle
+    assert 0 < threshold < n_entries
+
+    storage = get_storage_with_buffer_config(
+        min_batch_size={"directory_entries": threshold}
+    )
+    storage.storage = Mock(wraps=storage.storage)
+
+    for directory in directories:
+        storage.directory_add([directory])
+    storage.flush()
+
+    # We should have called the underlying directory_add at least twice, as
+    # we have hit the threshold for number of entries on directory n-2
+    method_calls = Counter(c[0] for c in storage.storage.method_calls)
+    assert method_calls["directory_add"] >= 2
+
+
 def test_buffering_proxy_storage_revision_threshold_not_hit(sample_data) -> None:
     revision = sample_data.revision
     storage = get_storage_with_buffer_config(min_batch_size={"revision": 10,})