The שולחן ערוך seems to contradict himself in this area. In או''ח סימן נה סעיף ו he says that it is ok to say קדיש whilst someone there is sleeping or saying שמונה עשרה and there are only 10 people in the room including that person. However, in סימן קכד סעיף ד he says that if there aren't 9 people responding to חזרת הש''ץ, it is almost a ברכה לבטלה. I am aware of three approaches to this issue.
- The דרישה in קכד,א says that the שולחן ערוך says almost for a reason. Less than 9 answering should be avoided but is OK. He also brings a proof from the הלכה in סימן נה סעיף ב that you can continue דברים שבקדושה when people walk out in the middle and you are left without a מנין. Based on that proof, he would be ok with any more than 6 people including חזן.
- מגן אברהם in סימן נה סעיף קטן ח says that we are not נוהג like the שולחן ערוך says in סימן קכד. He doesn't exactly answer for the contradiction we had but gives a פסק. Notably, he adds that מהר''ל אבן חביב was ok with any more than 6 including the חזן for חזרת הש''ץ but the מגן אברהם himself doesn't like it and says you need 9 including חזן.
- The שולחן ערוך הרב in סימן נה סעיף ז says that there is no contradiction because for חזרת הש''ץ you need 10 but only for קדיש and other דברים שבקדושה is less ok.
So there are a range of views. If no time pressure, wait for 10. If there is pressure, you can probably be מיקל for 6 people, but ask a Rabbi.