The short answer is forty. The longer answer is that "forty" means different things depending on whether you are playing at a kitchen table or registering a deck at an event, and that a Riftbound deck is really four separate piles of cards rather than one.

The short answer

• The Core Rules require a Main Deck of at least 40 cards.
• The Tournament Rules require exactly 40 in constructed competition.

So a casual deck can be larger than forty. A deck you register at a sanctioned event cannot. Almost everyone plays forty regardless, because a larger deck dilutes the cards you actually want to draw.

What a full deck is made of

Four separate components, only one of which is the "deck" most people mean:

The limits that shape a deck

• Three copies maximum Your Main Deck can include up to 3 copies of any one card name, and that includes your Chosen Champion, so you can run the champion plus two more copies.
• Domain Identity Your Champion Legend sets which Domains you may play. A card with one Domain needs that Domain in your identity; a card with several needs all of them.
• Chosen Champion Must be a champion unit whose champion tag matches a tag on your Champion Legend. Signature units do not qualify, even with a matching tag.
• Unique Some cards carry the Unique keyword, which restricts them during deck construction rather than doing anything during play.

What forty cards actually costs

Deck cost is dominated by a handful of printings rather than the bulk. Using live prices from our dataset, here is what the cheapest and most expensive ends of a set look like.

A functional deck built from commons and uncommons costs very little. What costs money is the specific chase printings a competitive list wants, which is the whole reason to check prices before building.