do you mean an encounter per long rest?I know where it came from. I don't know why it's still even being said. An encounter a day is 100% fine and it can be fun and interesting to every class involved while also being balanced amongst themselves.
I must strongly disagree. I ran a game with a cleric, a paladin, a warlock and a monk, and for a time I had usually encounter a day - but often those were big fights. The paladin was utterly dominating, could smite every round, an the cleric would toss around their most powerful spells. The monk and warlock paled.
So I changed the pace a bit, had multiple combats per day. Suddenly the paladin had to hold back and felt more reasonable.