How can I control the volume according to the number of people by making presets?
This is accomplished by a VCA, of which there are hundreds. Radio Design Labs makes small 'Stick-Ons' which can accomplish this for under $50. At the high end, we often use Peavey MediaMatrix to do complete audio processing of an entire facility (this is the theme park standard).
You can also purchase our AmpTraXX, which has both volume and EQ serial control.
All of these products are easily controlled from any of our show controllers.