Actually my uncle told me a person can only hear 4 melodies at once in the sense of four lines of melody, but you guys bring up some good points. I think there is another thing too - which is getting used to things. You can hear a pretty complicated loop with drums and synths, but then your ear becomes accustomed to it, so you need to make changes in rhythm or add a new element or something. Thats the bit where I am thinking about Jack's mixer's comment but carrying it a step further - to manage the listener's attention.
