hmm, i use one strand per whole -2 if i need to fill the gap but normally one-the key is to layer up; like a cake
start at the bottom with one layer and the next one goes on top of it to add fullness-too much tulle and yep it will stretch, did you add shoulder ties? that will help keep it on if its over-stretch-(what size did you make?) you can do simple shoulder ties and tie into a bow onto each shoulder or halter top, on piece of tulle tied at the neck (depends on age i wouldnt if they are babies or lying down do the shoulder ties then--
Does this help?