What is the Halachah about hearing multiple women singing, is it Kol Ishah or not? What if it is co-ed with a few women and a few men?
A related post reports the “famous Teshuva of the Sridei Aish, Rabbi Yaakov Yechiel Weinberg, that permits one to hear kol isha (a women's singing voice) when there are other people singing.” It can be found in Seridei Esh 2:8.
This is referred to in the Hebrew Wikipedia article on קול באישה ערווה
That article further reports that when Rav Weinberg visited Germany he found men and women singing Shabbos Zemiros together. This was allowed to them by Rabbis E Hildesheimer and S R Hirsch. Rav Weinberg supported this teaching with the views of those Rishonim who held that there was only a prohibition when the man had intention to enjoy the voice of the woman. Additionally, singing together meant that the voice of one individual woman could not be distinguished.
Rav Weinberg decided that one could rely on the views of these German Rabbonim in order to develop love of Torah etc. in boys and girls and to guard against assimilation. Rav E I Waldenberg does not support the view of Rav Weinberg.
Wikiyeshiva does not mention Rav Weinberg. It says that there is no difference if one or more women sing together. There are those (Chasan Sofer) who are unsure whether men and women may sing together insofar as the man does not distinguish the voice of the woman (and so it should not be forbidden). But Wikiyeshiva says that most other poskim reject this view and hold that one may not hear the voices of men and women singing together.
Conclusion: There are some authorities that have allowed hearing multiple women singing and some others hold that it is forbidden. CYLOR.