Much se usa para describir una gran cantidad de algo, many describe un gran número de cosas.

El uso de much y many
much mucho (singular!)
many muchos, muchas (plural!)
Ejemplos
a) I haven't got much time to wait for you again!
a) Do you have many friends in your town?
b) Eat as much as you like!
b) There are just too many people around; I cannot find Peter among them.
c) Much has happened during the last year.
c) Many people have left already for the day, because of the nice weather.