I don’t think about how many schools should be in each category, at least not past the likeliest categories. The main idea is to find the schools that really excite a student, that s/he would like to attend, and then to place them in the appropriate categories and make sure you have enough in the likeliest categories. Because as was discussed briefly in the thread about safeties, students and their families shouldn’t have (or create) a feeling that the best schools for a child are the most selective ones and that everything else is lesser-than. If a student’s favorite schools are highly likely for admittance, then great! There is no need to add on any that are less likely.
If I was dealing with a non-CC population, that’s how I’d probably start. But here at CC the focus tends to be so much on the Top X schools (whether overall or within a department) that almost all schools are less likely for admission. Thus, for the CC community, I recommend starting with looking at the likeliest options.
And as to your question, I think that 15 schools is on the heavy side for applications. But I’d have at least 1 guaranteed admission (either via an admissions chart or early timeline with rolling admissions), at least 2-3 in the extremely likely category, and no more than 7 total in the Possible/Less Likely categories. I think it is very hard emotionally on people if they are rejected to more than half of the schools that they apply to, and those categories have less than a 40% chance of happening (i.e. more likely to not be admitted than to be admitted). Thus, I would have the majority of schools be at least toss-up or above with respect to likelihood for acceptance.