A case: I am Chazan and finished the silent Shmona Esre, someone say me to begin the Chazara because there are already 9 who finished their Shmona Esre, but I don't believe him, I want to count by myself because I am worry to make the Chazara before the time.
When the Chazan finishes his silent Shmona Esre, he needs to check if 9 other men finished, and then he can start the Chazara. If someone says him that there are already 9, can he believe him. Apparently it is obvious.
But I ask myself, if one witness is believed for Isurim, by Svara when there is not Itchazek Isura, this is by Svara (see Yevamot 88a and Rashi). Because if one witness is not believed one cannot eat meals verified by someone else. When a man can check himself, the Svara is no more effective. So maybe he needs to check himself, So in a similar way, maybe that the Shats needs to count himself the 9 who finished the Shmona Esre. So what about one witness when one can check by himself, generally? And what about Chazarat Hashats?
I remember here some laws of Tefila.
Hilchot Tefila SA OC 124.6
כששליח ציבור חוזר התפלה הקהל יש להם לשתוק ולכוין לברכות שמברך החזן ולענות אמן ואם אין ט' מכוונים לברכותיו קרוב להיות ברכותיו לבטלה לכן כל אדם יעשה עצמו כאילו אין ט' זולתו ויכוין לברכת החזן
When the Shats prays the tefilla, the congregation, they need to be silent and to concentrate about the Berachot of the Chazan. If there are not 9 who concentrate about the Berachot, the Berachot are almost vain...