I have a question- when I play "Rule by Decree" plot. And 2 or more players have same number of cards. Shall all of them discard there hands.

No. The card says that "the player with the most cards in hand..." has to discard. If two players are tied for the highest number of cards, then no single player has the "most" cards. And since no one matches the play restrictions, the effect doesn't happen.

Thanks. I understand.