* Your Hand: You can play as many cards as you have in your hand that you can legally play (following the game's rules).
* Your Deck: Some cards in your deck (like "Pot of Greed") allow you to draw extra cards, potentially giving you more cards to play.
* Your Opponent's Cards: Some of your opponent's cards might limit the number of cards you can play (e.g., some monsters have effects that limit how many monsters you can summon).
* The Game's Phases: You can play different types of cards during different phases of the turn (Draw Phase, Stand-by Phase, Main Phase 1, Battle Phase, Main Phase 2, End Phase).
In general:
* You usually start with 5 cards in hand.
* You can only summon 1 monster face-up each turn (except for some special summoning effects).
* You can activate as many Spell/Trap cards as you have available (if you can legally activate them).
Therefore, there is no "set" number of cards you can play in a turn, as it depends heavily on the situation and what cards you have available.