Rule 402.2 · Hand
Each player has a maximum hand size, which is normally seven cards. A player may have any number of cards in their hand, but as part of their cleanup step, the player must discard excess cards down to the maximum hand size.
Ask the judge Pro
The life counter has an AI judge that answers your play citing the exact rules. It is part of Gatewatch Pro; the rules, Oracle text and rulings stay free.