Re: Limit to make Naruto Shadow clones
The typical clone technique would be an illusion of some sort; however, Shadow Clones are tangible, solid, mentally linked with their creator (they can wander off and learn stuff and their creator will automatically know it), that sort of thing. Supposedly, they're as tough as their creator, but that doesn't actually bear out, as shadow clones go *poof* as soon as they take any reasonable hit. They can also be easily dismissed from just about anywhere. There's a limit based on how much chakra (life energy, ki, etc) the wearer has, but loss of a clone isn't permanent; you can always poof up another one once your energy's recharged.
I would say Summon would be the way to go, simply giving the summoned duplicates only 1 Body (so they'll die if they take 2), and just say death = poof.