Official rule text
2.4 Each player’s hand can have a maximum of seven action cards. If a player ever has more than seven action cards, that actions during an action phase if all other players have player must choose seven cards to keep and discard the rest. passed during that action phase. a A game effect can increase or decrease the number of cards
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 2.4