Well, real life doesn't always have the simplicity of a story problem... I ended up getting over-ruled, and rather than filling the front room with balloons, we filled both bathrooms. I never got a count on balloons, as everyone just showed up with half open packs, etc. and we never organized enough to get a count.
I can share with you a few things that I learned:
1) 12 inch balloons are the way to go, then fill up space much better than the 9", definately worth the extra cost. 2) Don't bother with those clip things that make it so you do not need to tie a knot. You need the 10 seconds or so it takes to tie the knot to recover from the light head from blowing up the balloon before you start on the next one. 3) Go to the bathroom before you start... as once it is half full of balloons no one can use the facility.