I hate to limit kids in what they can put in their application if it was meaningful to them. But I do wish there was a way to more effectively convey to kids that quantity per se is usually completely irrelevant. And that, frankly, most AOs are only going to glance at the lower part of the list, and often give it no real consideration in the overall assessment, unless something really unusual or interesting stands out.
So I don’t know–how about like 6 slots to start, and then you can ask for 3 more in Supplement #1, then another 3 in Supplement #2, and so on. Maybe unlimited supplements if you really want them, maybe an ultimate cap, and the numbers are flexible. But the point would be to make clear to kids that typically the top few are going to be the ones that matter, and more than a few are not at all necessary.