In Card Hoarder there are normal and special decks.
Normal decks normally have 20 cards and are worth 1. Some normal decks are split across multiple sets, these will usually end in either A,B,C, etc. (i.e. The KH Days Opening Deck consists of KH Days Opening A, KH Days Opening B, KH Days Opening C)
As each is a separate deck, you can get multiple mastery rewards. (i.e. The KH Days Opening deck consists of 3 decks, so 3 lots of rewards)
An example of a normal deck
Special decks are worth 2 but can have any number of cards.
An example of a special deck