¡Cuánta razón!
Looks like, how much reason = that's SO right, isn't that what it means? Why not plural? Why not some other stuff (only half here sorry)
2 Answers
¡Cuánta razón! You could try: It's so right!
¡Cuánta razón tienes! ; ¡Tienes (mucha) razón! or ¡Estás en lo cierto! - You are (so) right!
This means, you are right.