Another option is to make a scheduled event and execute it for example every hour. Retrieve all users by the lastlogin attribute of the system.user entity and aggregate it. Place the 'count' in a custom entity.
$OneHourAgo = addHours([%CurrentDateTime%], -1)
[LastLogin >= $OneHourAgo][LastLogin < $CurrentDatetime]
This way you can keep up with how many users have logged at least once in the preceding hour.
You can use the after signin microflow to update your login count.