mtg what cards you can only have one in your deck

[Muk]

what cards you can only have one of in your deck?

[Arix]

Depends on the format. Vintage is the only format with a restricted list:

The following cards are restricted* in Vintage tournaments

Ancestral Recall
Balance
Black Lotus
Brainstorm
Burning Wish
Channel
Demonic Consultation
Demonic Tutor
Fact or Fiction
Fastbond
Flash
Frantic Search
Gifts Ungiven
Gush
Imperial Seal
Library of Alexandria
Lion’s Eye Diamond
Lotus Petal
Mana Crypt
Mana Vault
Memory Jar
Merchant Scroll
Mind’s Desire
Mox Emerald
Mox Jet
Mox Pearl
Mox Ruby
Mox Sapphire
Mystical Tutor
Necropotence
Ponder
Regrowth
Sol Ring
Strip Mine
Thirst for Knowledge
Time Vault
Time Walk
Timetwister
Tinker
Tolarian Academy
Trinisphere
Vampiric Tutor
Wheel of Fortune
Windfall
Yawgmoth’s Bargain
Yawgmoth’s Will

No other format has a restricted list, so you're allowed 4 of every legal card in that format.

[seba1983]

due to that restrict list most vintage decks run a very large number of singeltons in thier decks.

and there are casual formats like highlander or EDH where any coard other tahn basic lands and relentless rats can only be included once.

in EDH you play with 100 cards opposed to the normal 60. this led to a joke about vintage based on a missperception of the format:

q: what is the diffrence between vintage and EDH ?

a: 40 cards.