According to the Comprehensive Rules on shortcuts:
716.2a 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.
This rule is never contradicted in the Tournament Rules and should thus hold true to any play environment.
This rule states that a shortcut loop requires that the number of iterations of the loop be specified. Therefore, an arbitrary number of loop iterations (such as "a bunch") is never a legal shortcut. You should get in the habit of always specifying an exact number of loop iterations when declaring shortcuts such as this, and ask that your opponents do the same. No information about any player's plans is given away if the rule is applied consistently.
In your situation, I would simply say,
"You have to specify how many a bunch is."
This language ("you have to") shows that you're just requesting the shortcut rules be followed properly, not that you're necessarily interested in the exact number, or have a number in mind that would please you.