Each water dispensing device should have its own valve. That means one valve for every hot and one for every cold for each fixture or appliance.
If there is a problem (a leak, a malfunctioning unit) this allows you to cut off the problem device without losing functionality of other devices in the system.
For example, if you had a leaking toilet filling valve (inside the tank), you could turn the toilet supply off without losing the cold water in the sink and the tub in that same bathroom. If you had a delay in getting a part or doing a repair, this could be very important.
Having a separate cutoff valve near the dispensing device also allows a fast response to cut off the flow if there is a sudden leak. No running to the basement while the upstairs bathroom floor fills with water.