Does one need to wait for 10 men to answer to say kadish or kedusha or just majority (i.e. 6 ppl)?
For purposes of kadish, one can start with six alone (see Shulchan Aruch Ha-Rav 55:7), though it is preferable to wait for nine (with another who is still davening).
For chazaras ha-shatz one should wait for ten, though under extenuating circumstances it can be said even if there are nine (one still davening), and under very extenuating circumstances (for example where those who have finished must leave) six is enough.
See Mishnah Berurah 55:10-11 and 32; see also Teshuvos Ve-Hanhagos Vol. 1, no. 104.