There are many brachos where by listening to the bracha one can be yotzei his obligation for that bracha. If someone does do so, does that bracha count towards the quota of 100 brochos a day?
Shulchan Aruch (Orach Chaim 284:3) says that "one must pay attention to the berachos recited by those called to the Torah and by the maftir, and answer Amen to them, which will count towards the total of 100 berachos that one is short of on Shabbos."
That said, Mishnah Berurah (46:14) says that this is only ע"פ הדחק, where you don't have another way to do so. His source for this, Magen Avraham (46:8) citing Maharam Mitrani, is even more explicit: this works, he says, only if you don't have fruits or other ways to complete the 100 berachos yourself.