Q. See question above. Would the same answer apply even on Shabbos an Yom Tov?

A. Mibeis Yisroel (50 p. 65) rules that even then in principle one davening a voluntary Amida would count as a part of the minyan. However, he is in doubt if one can actually really apply it.

Horav Yitzchok Zilberstein Shlit’a mentions an added point not to complete a minyan ne davening a voluntary Amida, since many opinions maintain that a voluntary tefila should not be recited at all during Shabbos and Yom tov.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a