Well, that's the kind of question that could be discussed over, and over, and over... With no right answer.
And, even in that case, I see that she has some regret about not applying to more schools. One of the reasons she got so many offers is that she applied only to lower ranked schools, because she thought she was not competitive due to a low GMAT score. Since she got 6 offers from lower ranked schools, it might be very possible that she would get an offer from a higher ranked schools if she had applied to those. Now, she will probably never know for sure. But she is happy with the offers she has, so it is also not such a big deal.
I've seen many applicants regretting the decision to apply to too few schools. I've never seen an applicant regretting applying to too many schools. Remember that the acceptance rate of PhD programs in general is extremely low, and competition is very fierce. Even the strongest applicant will not be showered with offers. Ok, you may be a great applicant, but there are hundreds of great applicants out there. Overestimating your odds may be a costly mistake. As well as underestimating your odds.
So, I believe you should apply to many schools. But that doesn't mean the more, the better. Several things should be taken into consideration before applying to every school out there.
1 - How common is your research interest? If it is common, you will find a lot of schools and professors who are a great fit for you. If it's not so common, or very specific, you will find fewer schools which would be interested in you. In my case, marketing metrics have been a research priority for many years according to the American Marketing Association (AMA), so it was not so hard to find schools which seemed to be interested in that subject like me.
2 - How sure are you about the strength of your profile? If you are really sure about your competitiveness, you might be able to focus on schools at your level. For example, if you're sure your strength is suited for schools ranked Top 30, you could aply only to schools around that level. But it is extremely hard to assess your strength. So, if you believe your strength is Top 30, but you aren't so sure about that, it is probably a better strategy to apply not only to schools around Top 30, but also above that (Top 10, for example) and below that (Top 50, for example), covering your bases. In my case, people said I was an applicant for a top 25 school. But I really had doubts about it. I saw many strong points in my profile, but also a few weaknesses (like my age, my low GPA even if it was from a top school, and letters of recommendations from professors who are known in Brazil but not necessarily in the US). So, I focused my applications in the top 20-top 30 range, but also applied to schools ranked higher and lower than that.
3 - How much time do you have to prepare your application? Each school you add to your list is more work to be done. What kind of work? Read papers from the professors who are of your interest in each school (some schools may have several professors, each one with several interesting papers to read), and write a different Statement of Purpose for that school (you should not use a standard text for all schools, but I'll write more about it in the future), among other things that you shouldn't do in a hurry. I had a lot of time to prepare my applications, I applied to all of them before the deadline.
4 - How much money do you have saved for applications? Even credit card limits may be considered. Just the application fee is around $ 100 for each school. But total costs may be much higer, sending additional GMAT scores for each school, for example. I had money enough to apply to many schools, I believe it was not something I should be stingy with.
5 - How many schools are you really interested in? Please, only apply to a school if you'd be happy to go there for your PhD. It's annoying to see an applicant receive an offer, and then sound disappointed, planning to give up and try again next year. It's not just annoying, but a waste of time for the applicant, the school, and even other applicants which may be waitlisted because of that. I didn't get accepted by a "dream school" like Harvard, but I'd be more than satisfied to have offers from any schools I applied to. I will also write more later about school selection.
Even with all this, I think 5 is a really small number. I've seen a person who applied to only 1 school. But, then, it was an applicant who had great contacts with professors from that school and knew the chances were very good. I applied to 20 schools, but I was really risk averse, and that number is too much for most applicants. I calculated the average number of applications from a sample of 10 other business PhD applicants, and the average was 12 applications, with a minimum of 8 and a maximum of 18.