(note: the following is entirely my own opinion, and, as far as I can tell, is neither supported nor countered by the actual rules.)
I tend to rule that there *is* a limit, but that it's a relatively generous one. You can glamour up yourself, and an ally or two (assuming they fall into the category of someone who's made some kind of pact with you), and your pet cat, and then, as you try to stretch your power further to make your motorcycle look like a horse, I throw an appropriate compel at you - a changeling might find himself shifting more towards fey due to working with that much power at once; someone who has glamours through an item of power might find that he's attracted the attention of whoever gave him the item (and/or ordinary fey in the area), etc.
So, no making the army of mercenaries you hired all look like goblins. But a pair of ninjas? That could work.