Shulchan Aruch, Orach Chayim 124:4, in my own, loose translation:
When the leader repeats the amida (sh'mone esre), the congregation should be quiet, pay attention to the benedictions that the leader recites, and respond "amen". If there are fewer than nine paying attention to his benedictions, his benedictions are nearly in vain. Therefore, everyone should treat himself as if there were not nine besides for him and pay attention to the benediction of the leader.
Normally, women and minors don't count toward a minyan, a quorum of ten required for various things, especially d'varim shebikdusha, matters of holiness. (One minor may count, as outlined somewhere else in Shulchan Aruch, but let's ignore that fact for simplicity.) I wonder whether the same is true here, for the following two reasons:
- The Shulchan Aruch doesn't say explicitly that they need to be adult males.
- This sounds to me like a "someone has to be listening" thing rather than like a "we need a minyan here" thing. For example, the Aruch Hashulchan (:9) paraphrases the Taz (:2) to the effect that the necessary nine cannot include someone whose ears are stuffed up so he cannot hear, even though he counts toward a minyan.
Do women count toward this quorum of nine listeners? Do minors?