Good is an adjective. It modifies a noud.
Eat a good breakfast. (good = breakfast)
Freedom is good. (good = freedom)
Jimi is a good guitar player. (good = player)
Well is an adverb. It modifies a verb or adjective.
Eat well and be happy. (eat = well)
Jimi plays well. (play = well)
Many of has to refer to a specific group.
Many people like me. (We don't know how many)
Many of those people like me. (We know it's a portion of a specific gtroup)