Depending on where you run a promotion, you're sometimes required to offer an equivalent cash prize as an option. This can have the side-effect of the number of digital items in a promotion being artificially limited simply so that you don't have a situation where you're functionally writing a blank check for an unknown quantity of money; by limiting the number of digital "items", you also limit what you might need to spend on it.
It's dumb, but there is an actual rational reason why there can be an artificially imposed "limited supply" of a functionally infinite digital good.
In my own experience, this usually only applies to contests specifically. But I'm not going to swear there's nowhere that it applies to promotions in general, and I would in fact not be surprised in the least if there was such a place.
