Is this a local requirement?
Some shower valves have integrated valves built into the faucet already.
Valves are requirement for fixtures in the event of product failure, thus allowing the average user the capability to attempt to shut off water to source to shut off water.
In copper piping, you run one line and reduce down to the last three hots and colds to 1/2″.
Main line should be minimum 3/4″ from the main to the water heater and breaking down from there.
“Your best interest is secured by making the right decisions the first time.”