Collector Booster Sample Packs contain two cards per pack. For most releases, you can expect one rare/mythic (extended art or special treatment) and one common/uncommon. For some releases, such as Final Fantasy, you will will always receive two rares per pack. You also always get an ad card everytime (that isn't one of the two included cards) - if you want to go down the ad card iceberg, go here, here, here, here, and here.
You will receive twelve (12) total collector sample packs from a variety of releases as follows:
Three (3) Bloomburrow Sample Packs
Three (3) Modern Horizons 3 Sample Packs
Three (3) sample packs from the following pool of releases:
Commander Legends - Battle for Baldur’s Gate
Commander Masters
Doctor Who
Dominaria United
Fallout
Final Fantasy
Lord of the Rings
Phyrexia: All Will Be One
Wilds of Eldraine
Three (3) sample packs from the following pool of releases:
Aetherdrift
Duskmourn
Edge of Eternities
Lost Caverns of Ixalan
March of the Machine
Murders at Karlov Manor
Outlaws of Thunder Junction
Streets of New Capenna
Tarkir: Dragonstorm
The Brothers’ War