Steam Charts are giving overall maximum online players count for Age of Empire 2: Definitive edition. Age of Empires II: Definitive Edition - Steam Charts
However, it is not clear if there numbers include XBox service players. Technically, Steam is getting players nicknames and their availability from XBox gaming. But does XBox shares information on players numbers? And is the information about maximum 38 thousand online players for this game correct for XBox players as well?
This site shows only players who are playing the game through Steam.
No idea if the data for Xbox players are available anywhere