Tracks Blizzard employees across various accounts.
Wizards Jay
Really need a full list to see how many cards are new vs existing to know how much this will ultimately matter.
There's a breakdown at the end of this article: https://magic.wizards.com/en/articles/archive/magic-digital/what-mtg-arenas-jumpstart-historic-horizons-2021-07-26
For the 372 new-to-MTG Arena cards:
13 Mythic Rares
56 Rares
123 Uncommons
180 Commons
For the total of 767 non-land cards:
85 Rares
286 Uncommons
383 Commons
Wizards Jay
There's a breakdown at the end of this article: https://magic.wizards.com/en/articles/archive/magic-digital/what-mtg-arenas-jumpstart-historic-horizons-2021-07-26
For the 372 new-to-MTG Arena cards:
13 Mythic Rares
56 Rares
123 Uncommons
180 Commons
For the total of 767 non-land cards:
13 Mythic Rares
85 Rares
286 Uncommons
383 Commons