Case 1: Tournament Rules Magic: N + 15 cards, where N is the number of cards in your deck.
There is currently no way to exchange ownership of cards in Magic: The Gathering that is legal in tournament play. Cards that refer to fetching cards from "Outside the Game" like Living Wish may only search your sideboard, which is limited to 15 cards. The maximum number of cards you can own in a game is equal to the size of your deck + the size of your sideboard.
Case 2: Tabletop Magic, legal magic cards only: N + M, where N is the number of cards in your deck and M is your total collection of magic cards.
Per rulings on any of the cards that refer to "Outside the Game", any card from your collection may be fetched by these cards assuming they follow the criteria specified by the card.
In a sanctioned event, a card that’s “outside the game” is one that’s in your sideboard. In an unsanctioned event, you may choose any card from your collection.
Here's the combo we're going to set up and use in later stages.
- Tap City of Brass for mana.
- Cast Cloud of Fairies and untap City of Brass
- Cast Cavern Harpy and return Cloud of Fairies to your hand.
- Pay one life to return Cavern Harpy to your hand.
This nets you one mana of any color and returns to a previous game state. Repeat for any amount of mana you need.
- Cast Research, get four cards from your collection and shuffle them into your library.
- Cast Eternal Witness to return Research to your hand.
- Cast Man-o'-War to return Eternal Witness to your hand.
- Cast Cavern Harpy to return Man-o'-War to your hand.
- Pay one life to return Cavern Harpy to your hand.
You've now net +2 life and in the same game state as before. Repeat until your collection is in your library.
Case 3: Tabletop Magic, Ante Cards allowed: N + M, 1 * (Number of Opponents) after the game is over.
If you are playing with Ante rules and your opponents aren't cooperating with you and are deliberately trying to prevent you from owning as many cards as possible, the number is the same N + M from before, plus receiving each player's Ante once the game is over.
There are ways to own every card in every player's deck + collection using Ante, but due to the ability for a player to concede at any time, opponents would be able to prevent you from pulling off any combo that allows you to steal cards. The cards anted by opponents would then become yours after the game ends.
Case 4: Tabletop Magic, Ante Cards allowed, opponents cooperating with you: all cards in all players respective collections.
Using only Ante cards in tabletop magic and with your opponents cooperating with you, it is possible to own every card in every player's collection using a handful of new cards.
- Use the same board state as Case 2, but also have Hive Mind on the battlefield. Cast Research until all of your opponents collections are in their library.
- Have a method to create infinite token copies of a creature. For the sake of argument, use Rite of Replication to make your copies. Recur it back to your hand in the same process as Research from Case 2.
- Create a number of token copies of Tempest Efreet equal to or greater the number of cards owned by each opponent.
- Create a copy of Jeweled Bird for each player, Harmless Offering a copy to each opponent (using your infinite mana and recursion from step 1 to do so), and have each player use the token to put their anted card into the graveyard. The token of Jeweled Bird ceases to exist in Ante as tokens can only exist on the battlefield.
- Cast Time Walk, cast Sway of the Stars, and while Sway is on the stack also cast Teferi's Protection. Your opponent's entire collection is now either in their hand or library, your board is unaffected due to Teferi's Protection.
- Once your permanents phase in at the beginning of your next turn, use your copies of Tempest Efreet to steal your opponent's hands. These tokens disappear once they enter your opponent's graveyard. (Note: we're using Time Walk so that opponents don't get another draw step before you can start stealing cards, and would thus have to discard to 7.)
- As your opponents draw their first card every turn, use another Tempest Efreet copy to steal that card from them.
- (Optional) If an opponent owns more cards than you, use Timetwister and your infinite recursion from step 1 to continuously shuffle your graveyard into your library so that you don't run out of cards.
Case 5: Tabletop Magic (We're gonna need a bigger table), ante/silver bordered cards allowed, all members of humanity cooperating with you: all cards in existence either owned by someone who is reachable by phone, and all cards currently in booster packs
Additions needed from Case 4:
- Better than One needs to be in your deck
- Booster Tutor needs to be in your deck
- An infinite real-world money combo (Combo of choice left to the reader)
- Have the same board state as Case 4.
- Use Better than One to make every opponent find a partner, but fail to find one yourself (it's important to fail to find a team mate, as your combo does not work with N-Headed Giant partners, which players recruited via Better than One become).
- Recruit every human being on the planet (since humanity is cooperating with us, that means that people who own MTG cards but are out of contact with humanity would come back into contact to make sure they can give us their magic cards using a convoluted in game combo using multiple illegal cards).
- Use the combo from Case 4 to make them fetch and then steal all of their cards.
- Destroy Hive Mind in some way, and use your infinite money combo alongside your infinite copies of Booster Tutor to purchase and open every extant Magic: The Gathering product in existence. Be sure to destroy Hive Mind because all these people are being nice and helping you steal their cards, and it would be rude to make them also buy the packs for you.