Even though the Galaxy A51 is yet to go official, Samsung has listed the device in its Android security update bulletin and scheduled it for quarterly security updates. Yes, the Galaxy A51, despite being a newer device, is set to get a security update every three months while the Galaxy A50 is on a monthly schedule.
However, it's possible the Galaxy A51 will get monthly updates despite what Samsung's official schedule might say. The Galaxy A50 was also slated to get quarterly security updates at launch, but Samsung released a new update for the device every month in most markets and officially shifted it to monthly updates later on. The Galaxy A51 will hopefully get similar support once it goes on sale, which should be sometime later this month following the phone's official unveiling on December 12.
Even if the Galaxy A51 doesn't get monthly updates, those who purchase the device will be able to enjoy the latest version of Android out of the box. The Galaxy A50's Android 10 update is currently slated to come out sometime in April, along with a number of other 2019 Galaxy A series phones. The Galaxy A51 will also be eligible to get Android 12, while the Galaxy A50 will end its life on Android 11 unless Samsung changes its long-standing policy of only providing two major OS updates to every Galaxy device.