There are ten playable decks in total:
Gideon Jura - "Wielding Steel"
Sorin Markov - "Bloodhunger"
Tezzeret Agent of Bolas - "Machinations"
Nissa Revane - "Guardians of the Wood"
Chandra Nalaar - "Unquenchable Fire"
Garruk Wildspeeker - "Apex Predators"
Jace Beleren - "Realm of Illusion"
Koth of the Hammer - "Strength of Stone"
Sarkhan Vol - "Dragon's Roar"
Kiora Atua - "Denizens of the Deep"
...
To say that players are able to fully customize their decks would be a misnomer as you're only allowed to used the 16 additional cards for each deck. Additionally, one doesn't have any control over the amount of land in their deck. What the new game does allow is for players to remove cards from the core of their deck, unlike the previous installment.