Shalom Aleichem Harav!
Q. I daven in a minyan that davens nusssach sefard; most of the mispallilm are not well versed with the nussach and revert to nussach ashkenass. The question is regarding the 13 attributes by tachanun; does it need to be recited by the whole minyan or at least 10 ?
Since the tzibur isn’t accustomed we usually have at most 4 people saying the middos.

A. Tur (O.H. 565) quotes Rav Nosson that the 13 midos require a minyan for their recitation, and wonders why, since it is not a “dovor shebekdusha.” However, Beis Yosef (ibid.) citing Rashbo asserts it is a most holy recitation, and therefore requires a minyan. His argument emanates from the origins of this unique tefila, as our sages teach; Hashem covered Himself alike a shliach tzibur and demonstrated to Moishe, how the people should pray, so He will surely answer them. Shulchan Aruch (ibid. 5) rules accordingly. Mentioning that without a minyan they cannot be recited as a tefila and supplication, but they can be recited as one reading the Torah. Mishna Berura (ibid. 12) explains that he should read it with the taamim an intonations usually uttered then.
Poskim disagree, whether you require at least six people to recite the attributes at the same time, or just the mere presence of a minyan suffices, and they may be recited by others a bit latter when they reach that place in tefila.
Horav Shlomo Miller Shlit’a advises to make short the recitation, and just mention the thirteen midos without the rest of the accompanying oration, so everyone will say it at the same time.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a