Q:

How do I interrupt a loop after multiple iterations?

This is a question regarding infinite loops and shortcuts at Competitive REL.

Here is a potential scenario.

Both my opponent and myself are at 20 life each.
I have no creatures on the battlefield. 
My opponent has a  Kiki-Jiki, Mirror Breaker and a  Warstorm Surge on the battlefield.
I have a  Rakdos Charm in hand.
My opponent plays  Deceiver Exarch and begins a loop where he uses Kiki-Jiki to copy the Exarch, and uses each new Exarch to untap Kiki-Jiki, dealing me 1 damage every time he does it.
If I cast  Rakdos Charm when the 18th Exarch token's trigger is on the stack, I win.
If I cast it at any other time, I lose.
If my opponent knows that I'm going to cast it than he won't make the 18th token, and he'll simply attack and kill me.

Now, the question is:  
If my opponent proposes a shortcut, saying

"I'll make 500 tokens and kill you."

Can I accept the first 18 iterations of that shortcut and react by saying 

"After the 18th token enters the battlefield and its ETB trigger goes onto the stack, I cast Rakdos Charm."

, or do I have to propose a new shortcut for the 18 iterations, potentially giving my opponent information?

A:

Yes, you can accept only the first 18 iterations. After he proposes the number of iterations for the loop (500 here), you can choose a lower number of iterations, and both players have to abide to those iterations.
The rules for this are described in 719. Taking Shortcuts. A few quotes from that section:

719.2a At any point in the game, the player with priority may suggest a shortcut by describing a sequence of game choices, for all players,
  that may be legally taken based on the current game state and the
  predictable results of the sequence of choices. This sequence may be a
  non-repetitive series of choices, a loop that repeats a specified
  number of times, multiple loops, or nested loops, and may even cross
  multiple turns. It can’t include conditional actions, where the
  outcome of a game event determines the next action a player takes. The
  ending point of this sequence must be a place where a player has
  priority, though it need not be the player proposing the shortcut.
719.2b Each other player, in turn order starting after the player who suggested the shortcut, may either accept the proposed sequence, or
  shorten it by naming a place where he or she will make a game choice
  that’s different than what’s been proposed. (The player doesn’t need
  to specify at this time what the new choice will be.) This place
  becomes the new ending point of the proposed sequence.
719.2c Once the last player has either accepted or shortened the shortcut proposal, the shortcut is taken. The game advances to the
  last proposed ending point, with all game choices contained in the
  shortcut proposal having been taken. If the shortcut was shortened
  from the original proposal, the player who now has priority must make
  a different game choice than what was originally proposed for that
  player.

