Say I've been playing for a while, and I've got a nice little stack of story tokens. I want to set up one of those big climactic battle scenes were the giant robot gets picked apart by the heroes, gradually destroying various parts of it until it finally gets brought down.

I start off with the primary villain, "Giant Robot". I make its abilities center on moving around and defending itself. With story tokens, I buy, say, three non-person characters: "Energy cannon head", "Claw right arm," and "Buzzsaw left arm." Each one comes with its own free conflict: "Goal: Destroy Head", "Goal: Destroy Right Arm," and "Goal: Destroy Left Arm"

I've got four characters on the board, so I encourage the other players around the table to all bring in characters who are interested in opposing the robot.

Is this a legal play? Is there a limit to the number of story tokens that can be spent to bring in more characters?

Yes, it's legal. No there's no limit.

Do you actually have to ask this question? Is there some passage in the rules that makes you doubt that this would be legal?

Yes, I felt I needed to ask, because my readings of the rules seem to have missed other things in the past.

Like the bit about Exemplar conflicts; I was expecting to find the rules under exemplars, and instead it was under claiming.

Not only is it legal, but it is totally awesome.

I think I understand why you're asking though. I can see how this could be abused horribly, by augmenting characters in this way. This isn't really any different than, say, spending your story tokens to bring in thirty unique superheroes in a single scene. In which case I think you're looking more at a social contract issue, as all the other players turn their icy glares on you for completely missing the point of the game. Such lameness could easily ruin everyone else's fun.

To summarize, there are really two rules of thumb for what is permissible in a Capes game:
1) Is it legal?
2) Is it awesome?

(Point 2 is implied by the veto rule.)

FWIW, in actual play, I consistently play less characters per scene than the other players in my wednesday group.

My anecdotal evidence is that my characters are more central to the story-line than other spotlight characters. I can get into my theories about the whys and wherefores, but it's sufficiently mathematical and dense that it's probably worth spawning a separate thread for it if there's enough interest. It's sort of tangential to the question of what the rules allow.

I'm interested!

Yeah, I'm very interested! I really like the tactics-and-strategy chapter in the rule book, and believe that reading what choices you can make and how it will affect you makes it a lot easier to understand how you can use the rules to get what you want.