A person is obligated to pay attention to chazaras hashatz , and answer amein to each beracha. If there are not nine male adults listening , the chazan’s berachos are considered to be virtually in vain. When a person pays attention to the entire chazaras hashatz, he receives reward as if he has davened a second time.
If a chazan suspects that there will not be nine people listening, he should stipulate before beginning that if that occurs, his shemoneh essrei (i.e, the chazaras hashatz) should be considered a voluntary tefillah.(And thus, it would NOT help on Shabbos,Yom Tov, nor at any mussaf, which cannot be a voluntary tefillah.