Riftwatchers will be a mini card set, not a fully set like Untamed or Chaos Legion. Dice was the first mini set and Riftwatchers will follow very closely to that. I believe there were just around 20 cards in Dice, so I'd guess around the same for Riftwatchers.

Although, Dice was mainly made up of summoners for most of the Rare cards. We don't know if that will be the same for Riftwatchers or not.

Also, Dice was only able to be bought by DEC. The price was 3000 DEC ($3 since DEC was mainly $0.001 during the sale) at the time. I'm sure Riftwatchers will be a bit extra, and may require vouchers to be used with it.