Squashed commit of the following:

commit d6dcf98d7445cbdc2a036deca57207c14bc354fc
Author: Dan <14043624+delivrance@users.noreply.github.com>
Date:   Wed May 12 09:35:18 2021 +0200

    Rename get_chat_onlines to get_chat_online_count

commit 21ff2a39d856ebc939ce9b15810198c82a9c23c6
Merge: 808c629f 29701a3a
Author: Dan <14043624+delivrance@users.noreply.github.com>
Date:   Wed May 12 09:32:59 2021 +0200

    Merge branch 'master' into get-chat-online-count

commit 808c629f43b185bc0df8337a82f5ecc860bbdb94
Author: Andriel Rodrigues <andrielkogama2@gmail.com>
Date:   Wed May 12 04:28:53 2021 -0300

    Add get_chat_online_count method (todo) (#654)

compiler/docs/compiler.py

@@ -222,6 +222,7 @@ def pyrogram_api():
             set_slow_mode
             mark_chat_unread
             get_chat_event_log
+            get_chat_online_count
         """,
         users="""
         Users

pyrogram/methods/chats/__init__.py

@@ -53,6 +53,7 @@ from .unban_chat_member import UnbanChatMember
 from .unpin_all_chat_messages import UnpinAllChatMessages
 from .unpin_chat_message import UnpinChatMessage
 from .update_chat_username import UpdateChatUsername
+from .get_chat_online_count import GetChatOnlineCount
 
 
 class Chats(
@@ -92,6 +93,7 @@ class Chats(
     DeleteUserHistory,
     UnpinAllChatMessages,
     MarkChatUnread,
-    GetChatEventLog
+    GetChatEventLog,
+    GetChatOnlineCount
 ):
     pass

pyrogram/methods/chats/get_chat_online_count.py

@@ -0,0 +1,46 @@
+#  Pyrogram - Telegram MTProto API Client Library for Python
+#  Copyright (C) 2017-2021 Dan <https://github.com/delivrance>
+#
+#  This file is part of Pyrogram.
+#
+#  Pyrogram is free software: you can redistribute it and/or modify
+#  it under the terms of the GNU Lesser General Public License as published
+#  by the Free Software Foundation, either version 3 of the License, or
+#  (at your option) any later version.
+#
+#  Pyrogram is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public License
+#  along with Pyrogram.  If not, see <http://www.gnu.org/licenses/>.
+
+from typing import Union
+
+from pyrogram import raw
+from pyrogram.scaffold import Scaffold
+
+
+class GetChatOnlineCount(Scaffold):
+    async def get_chat_online_count(self, chat_id: Union[int, str]) -> int:
+        """Get the number of members that are currently online in a chat.
+
+        Parameters:
+            chat_id (``int`` | ``str``):
+                Unique identifier (int) or username (str) of the target chat.
+
+        Returns:
+            ``int``: On success, the chat members online count is returned.
+
+        Example:
+            .. code-block:: python
+
+                online = app.get_chat_online_count(chat_id)
+                print(online)
+        """
+        return (await self.send(
+            raw.functions.messages.GetOnlines(
+                peer=await self.resolve_peer(chat_id)
+            )
+        )).onlines