Halakhah к Йешайау 1:28

If there is only an exact minyan, in the synagogue it is prohibited for any of them to leave. Of one who leaves it is said:²⁴Isaiah 1:28. "Those who abandon God will be consumed." But, if ten will remain, the rest may leave if they have already heard Barechu, Kedushah and the Kaddishim after Aleinu. In the event that less than ten were left they may conclude the prayers, even with less than ten (provided that they are a majority of a minyan). If the chazzan had started to repeat the Shemoneh Esrei, he may complete the entire repetition of the Shemoneh Esrei and the congregation may say Kedushah, and he (the chazzan) also says: Elokeinu veilokei avoseinu barecheinu baberachah etc.²⁵These are the Kohanim blessings. However, the kohanim do not bless the congregation.²⁶Lit. “Raise up their hands”—the position of the Kohanim while blessing the congregation. Also, the Kaddish that follows the Shemoneh Esrei is not said²⁷According to Mishnah Berurah, both the half-Kaddish and the whole Kaddish are said. because these are different subjects. If they began the Torah reading with ten men and some [congregants] leave, the reading may be completed, but no additional [aliyahs] are permitted, and no one is called for maftir, but, rather, the last of the allotted olim reads the Haftarah without the berachos.

If the chazzan started kaddish or kedusha with 10 people and a few left he can finish the kaddish and kedusha that he started as long as the majority of them remain. Rem"a: Nevertheless, it is a sin to leave. And regarding that, the verse says: "...and those who forsake the Lord shall perish" (Isaiah 1:28) [Yerushalmi]. But if there are 10 people remaining it is allowed [Mordechi in the last chapter of Tractate Megillah].