A sibling’s child is in the process of applying to BS for next Fall. However needs virtually full FA. The dilemma is that the child is gravitating to very small schools. I have advised adding in larger schools with bigger endowments. She likes schools like Madeira, Grier and Emma Willard but also has co-ed on her list . How many is too many when casting a wide net for FA? She is a straight A student or close to it with some very nice distinguishing EC’s. No SSAT yet. So I think IF her SSATs are strong she will be at least competitive at most schools. However she is not generally a strong test taker (standardized) so we will see. I have advised them to start with 15 schools and whittle them down after the scores come in. Any other thoughts welcome! 
FA strategy should be to have a very good back up school! Apply to only BS’s that the student would actually attend if given money. So # of schools will depend on the student. I would focus on great fits especially schools that match strong EC(s) or sports. Also look outside your area- less competition. Expressing a strong interest in a school can help when it comes down to FA and yield- BS not going to give money out to kids they don’t think will come.
My take on the wide net is that it is more about selecting different types of schools. If you apply to 20 very similar schools, chance is that the result can be very similar compare to applying to 10 different schools
Every students situation is different. If the student is coming from a great school but just wants to go to BS, that’s different than a student that has no real options in their hometown. How to advise one, would differ from the other. In both cases, I wouldn’t want the child ending up at a school they hate- even with FA. Keep an open mind when looking at schools. Pick those you love- Plan for the worst, pray for the best!
Thanks @SculptorDad and @copperboom Fortunately there an excellent public option. However, the school is very very large and the feeling is that it is those other benefits like smaller class size, more diverse academic offerings and the ability to do ones outside activities all in one place.
What kinds of activities?
@CaliMex music, choir, soccer etc
We’ve done this only once and I’m no expert, but I think that when you need full FA there is no such thing as too many (provided the task of handling multiple applications doesn’t end up affecting their quality, of course). I also agree that she needs to apply only to schools she would love to attend if accepted. Expanding the geographic region will likely help.
When my daughter applied, the largest amount of FA came from the smallest school. The amounts also varied quite a bit.
I second what @GoatMama said. I have a child at BS on substantial FA and if we had it to do over again we would apply to more schools. In my mind there is no such thing as “too wide” of a net. That being said I wholeheartedly agree that applications should only be made to schools where the student and the parent(s) would accept an offer of admission. Don’t let your child apply to a school you won’t send them to - sets you up for the terrible situation where they come to you excited about their acceptance and then you have to tell them you won’t let them go there.
The schools that you listed that are on her top list are not as popular as a lot of other BS mainly because of the single sex factor. Now while Im a proponent of single sex, I think they will be a better bet for your niece especially the one with a first choice letter since most girls who look at single sex schools wind up choosing co-ed.