Quick take
CS8 suits 1–3 bathrooms at 9 GPM service; CS9 targets 2–5 bathrooms at 11 GPM; CS10 targets 4–6 bathrooms at 15 GPM. Choose from simultaneous demand, not bathroom count alone.
CS8
RKIN lists CS8 for 1–3 bathrooms, 9 GPM service / 12 GPM peak, 8-inch tank width and 1-inch connections.
CS9
CS9 is listed for 2–5 bathrooms, 11 GPM service / 17 GPM peak, 9-inch tank width and 1-inch connections.
CS10
CS10 is listed for 4–6 bathrooms, 15 GPM service / 23 GPM peak, 10-inch tank width and 1-inch connections.
Which one offers the best margin?
CS9 is a strong middle tier for many homes; CS10 is more appropriate when four or more bathrooms, large showers or simultaneous demand make 11 GPM feel tight.