The Redmi Note 9 Pro Max appears to have been largely forgotten about by Xiaomi. The device has now received a second MIUI 12 Stable Beta update, with the first having arrived in September. There is no word on the arrival of a Stable MIUI 12 update for the Redmi Note 9 Pro Max yet, though.
https://www.notebookcheck.net/The-Xiaomi-Redmi-Note-9-Pro-Max-remains-without-a-stable-MIUI-12-upgrade-in-India-as-another-Stable-Beta-update-arrives.510719.0.html