How many major sins are there, and are backbiting and mockery not among them, even though Imam Al-Dhahabi mentioned in his book "Al-Kaba'ir" that their number is seventy, and did not mention backbiting and mockery?

Major sins are every sin for which a legal punishment (حدّ) has been prescribed in this world, or a threat of punishment in the Hereafter, such as torment, wrath, retribution, warning, or a curse mentioned in the Quran or Sunnah. Backbiting (Gheebah) and mocking the religion are among the greatest major sins, even if Al-Dhahabi did not mention them, due to the threats of punishment against them in the texts of revelation. Major sins are not limited to a specific number, and may reach more than four hundred and sixty major sins.