common, here is a more specific Q&A regarding number of kaddishim:
Q; May one sign-up to learn the same mesechta mishnayos in memory of two different people who recently died, and only learn it once or does he have to learn the mesechta twice? While R’ Moshe (IG”M Y”D 1:254) rules that by Kaddish at least one Kaddish daily must be recited for each individual niftar, I am not sure if this same concept applies by learning mishnayos. R’ Ephraim Glatt
Answer: Rav Willig- Kitzur Divrei Sofrim Perek 55:246-7 quotes the Ra’anach who says that one kaddish can be said for two people because the zichron can be for two people. Divrei Sofrim suggests that this leiniency is only when the kaddish is for a parent and then a second person is added on. When it comes to mishnayos, it is even more important to learn twice.