look at duplicating movie clips.
for max ammount on stage set parameters for y, so if the mc goes off the top of the swf screen then it would remove itself from the swiff. When you create a new duplicate, increase the count number. But before you duplicate a clip make sure the number you have been incrementing is not greater than the max ammount. Make sure to decrease the incrementing variable when you remove a movie clip.

I havn't done much with sin or cos but if you create a movie clip that sets its y to random number in onClipEvent(load) then when you duplicate a movieclip, it will inherit those properties and move according to however you specify.