How strong do you want the balloon belly to be?  If it is for decorative purposes with no manhandling, six layers might be enough.  To be safe, twelve. 
Grocery bag?  Well, in kraft paper there are various thicknesses.  The thin is not much different from newspaper.  It is useful to do alternate layers with this or yellow pages or something, just to help see where you've covered.  The thicker type builds strength very quickly and the alternate of bond (office) and kraft (or even red rosin) makes a very strong skin.
For considerable strength, you can lay a thin (1-2mm) layer of fine pulp over the form.  Amazing.
Over a plasticine nose-piece?   Three would probably be enough.  When you remove it from the plasticene, you can always add some more - which don't need to be neat - inside before attaching it to the head.

Aluminium foil as an armature?   Remember that it's not all that strong.  It is quite good for giving shaping.  The tighter and harder you make it, the stronger it will be.  You can make a general shape with it, cover it with masking tape and add more bits to shape wherever you need it.  It can be cut, pressed, hammered, added to, taken from  . . .. quite versatile in fact.   If you combine it with wire, bits of wood, tubing etc. etc. you can solve a myriad of problems.
DavidO

Thanks! I'll look for Doris Rockwell's book.

Um, these aren't gonna be very big. Twelve inches high approx. I'm starting small.