diff --git a/src/DocumentStore.php b/src/DocumentStore.php
index 20a6e56..2cddfc1 100644
--- a/src/DocumentStore.php
+++ b/src/DocumentStore.php
@@ -143,4 +143,12 @@ public function filterDocs(string $collectionName, Filter $filter, int $skip = n
      * @return array
      */
     public function filterDocIds(string $collectionName, Filter $filter): array;
+
+    /**
+     * @param string $collectionName
+     * @param Filter $filter
+     * @return int The number of documents
+     * @throws UnknownCollection
+     */
+    public function countDocs(string $collectionName, Filter $filter): int;
 }
diff --git a/src/InMemoryDocumentStore.php b/src/InMemoryDocumentStore.php
index f2174b9..c0661f9 100644
--- a/src/InMemoryDocumentStore.php
+++ b/src/InMemoryDocumentStore.php
@@ -572,4 +572,21 @@ private function isSequentialArray(array $array): bool
 
         return \array_keys($array) === \range(0, \count($array) - 1);
     }
+
+    /**
+     * @inheritDoc
+     */
+    public function countDocs(string $collectionName, Filter $filter) : int
+    {
+        $this->assertHasCollection($collectionName);
+
+        $counter = 0;
+        foreach ($this->inMemoryConnection['documents'][$collectionName] as $docId => $doc) {
+            if ($filter->match($doc, $docId)) {
+                $counter++;
+            }
+        }
+
+        return $counter;
+    }
 }