Actually 5-6 (I use 10 everytime to be sure) blocks of C4 will kill a Jugg, and at the same time, not individually. They key is placement, and how you put the C4 down. Dont put them spread out in a small area, place them all in THE EXACT SAME SPOT, dont move your cursor and just repeatedly press the PLACE C4 Button appropriate for your platform. I know for a fact this works because I do it almost every day.
Key places where this works is in the CENTER of the floor in a small hallway, or right inside of a doorjamb you plan on luring them through, the wall of a small hallway, anywhere you can predictably get them to move within say 2 feet of the C4 packs works fine.
This tells me, and therefore you, :) that C4 DOES stack.
NOTE: WHILE YOU ARE PLACING THE C4 it is 'live'. So, if an enemy is shooting at you, and one of their stray bullets hits your C4 AS IT IS LEAVING YOUR HAND TO BE PLACED, the pack will blow and kill you. (friendly fire applies as well, as does any explosions friendly or otherwise that interfere AS YOUR PLACING THE C4)
THAT SAID: once the C4 is placed it becomes stable, and no enemy fire, explosions, ect, will blow it. So you can safely booby trap a room with C4 all over the freaking place, and get into a massive firefight in that room, relativly safely. Once its placed and still, the only thing that will blow it is your clacker (or double tapping reload, which is a 'hotkey' for the clacker.)