“but the number 40 in terms of pple, do you know a mekor? “
So many mystical events took place in Jewish history, or are connected to matters that are of a supra-natural (not supernatural) essence involving the number 40, that I suppose that is why the number was chosen by those who ascribe to it some koach. I would think that the number 8 would be likewise thought of in that way. I have no mekor for this, it is simply something that I have observed in my community.
And the answer to the question – well, what happens if the desired result is not achieved? Firstly, Hashem works on His cheshbon and timetable, not ours. And second, we are still showing hishtadlus and being pro-active in some manner. Can it ever be BAD for forty women to be mafrish challah on the same day and make a bracha on this MITZVAH of taking challah, in the hope that in zechus of doing the mitzvah with a united mindset that it will accrue to the benefit of one who is too sick to do the mitzvah herself? Is it neged halacha for a woman to go into a mikvah when she is pregnant, so that she feels a level of kedusha (much as does a man who is toveil regularly and has NO achrayus to ever do so and is not mevareich when he goes to the mikvah) so she goes into labor with a more positive mindset? The mind is a powerful tool. If one believes that her actions will lessen her difficulty in labor, that mindset alone will help her somewhat.
And if a childless woman has to go to the mikvah ANYWAY, does it HURT her in any way to go in after a pregnant woman? How many women get pregnant after adopting a baby? There is a sound medical reason for this well-documented phenomenon. Perhaps a similar type of relaxation (mentally, and therefore also physically) occurs when a woman truly believes that she might become pregnant doing this segulah. And if it does not work, she can always say, it was not meant to be at this time. It doesn’t take away from her approaching the mitzvah of tevilah with even more kavannah than usual.
Even our chachomim believed in the koach of talismans, cameos, and amulets. I personally never understood this, it seems like l’havdil elef havdalos a form of A”Z, but what do I know?