As we know, a Minyan – ten men above Bar-Misva age – must be present to allow the recitation of certain portions of the prayer service. These include Kaddish, Barechu, Birkat Kohanim, and the repetition of the Amida.
Sometimes it happens that ten men are present, but they are not all able to respond to the Hazan. Quite often, especially in small Minyanim, the Hazan and several congregants complete the Amida before the others, such that ten men are present, but fewer than ten would be able to answer to Kaddish, in the case of Arbit, or the repetition of the Amida, in the case of Shaharit, Minha or Mussaf. Must the Hazan in such a case wait to begin the repetition of Kaddish until ten men are able to answer, or may he begin even if some of the ten congregants are still reciting the Amida?
The Shulhan Aruch, in discussing the laws of Kaddish (Orah Haim 55:6), writes that Kaddish may be recited as long as ten men are present, even if one of them is reciting the Amida, and even if one of them is sleeping. He then writes that in a case where one of the ten congregants is reciting the Amida, it is preferable for the other nine to wait so that he can earn the Misva of answering to Kaddish. Essentially, however, Kaddish may be recited even if fewer than nine men are answering to the Hazan, as long as ten men are present. Hacham David Yosef, in his Halacha Berura, clarifies that even if as many as four of the ten congregants are praying the Amida, Kaddish may be recited, since six – the majority of a Minyan – are able to answer (including the Hazan himself). If more than one of the ten people are sleeping, Kaddish may not be recited, but if two, three or four people are still reciting the Amida, then although it is proper to wait for them so they can fulfill the Misva of responding to Kaddish, the Hazan may begin Kaddish even while they are still praying.
Later, however, in discussing the laws of the repetition of the Amida (124:4), the Shulhan Aruch appears to take a more stringent stance. There he writes that everyone in the synagogue must pay close attention to the Hazan’s repetition and answer "Amen" to all the Berachot, and he adds that if there are fewer than nine men paying attention, "the blessings are close to being blessings in vain." The Shulhan Aruch writes very clearly that nine men must be listening and responding to the Berachot of the Hazan’s repetition of the Amida.
Hacham Bension Abba Shaul (Israel, 1924-1998), in his Or Le’sion (2:5:6), brings different approaches to reconciling these two rulings of the Shulhan Aruch. Some claim that the Shulhan Aruch rescinded his ruling in Siman 55, and concluded that nine men must be answering the Hazan both during Kaddish and during the repetition of the Amida. Others, however, including Rav Schneur Zalman of Liadi (1745-1813), in Shulhan Aruch Ha’Rav, explain that the Shulhan Aruch distinguishes in this regard between Kaddish and the Hazan’s repetition of the Amida. When it comes to Kaddish, it suffices if even only five men answer, while the other four recite the Amida. When it comes to the Hazan’s repetition, however, the Shulhan Aruch requires nine men listening and responding. This is also the opinion of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909).
A third view is that of Hacham Ovadia Yosef, in Halichot Olam, as cited by his son, Hacham David. He notes that the Shulhan Aruch did not say that the Hazan’s Berachot are actually considered to have been recited in vain if fewer than nine men are listening and responding. Rather, the Shulhan Aruch said that the Berachot "are close" to having been recited in vain. The repetition is valid if even only five are listening and responding, and the Shulhan Aruch was merely impressing upon us the importance of having a Minyan of people listening and responding.
In light of these different opinions, Hacham Bension ruled that since some Poskim understood that the Shulhan Aruch retracted his earlier ruling, and requires nine men listening even for Kaddish, it is proper to ensure even for Kaddish that nine people are listening.
In practice, however, the accepted custom is to follow the view which distinguishes between Kaddish and the repetition of the Amida. And so at Arbit, we allow the Hazan to begin Kaddish after the Amida even if there are only five men answering. For the repetition of the Amida, however, we wait until nine men have finished the Amida, unless this would cause a very long delay in the service. For example, if there are only ten men present in the Minyan, and there is one who recites a very lengthy Amida, the congregation may rely on Hacham Ovadia’s lenient ruling and have the Hazan begin the repetition even though fewer than nine men will be answering.
Practically speaking, it is advisable when a small Minyan assembles to request ahead of time that everybody should finish the Amida by a certain time, in order to avoid this problem.
Summary: Kaddish may be recited if ten or more men are present even if only five are able to respond, because the others are reciting the Amida. Therefore, at Arbit, the Hazan may begin reciting Kaddish after the Amida already when five others have finished the Amida, though it is preferable to wait for the others so they can fulfill the Misva of answering to Kaddish. At the other prayer services, when the Hazan repeats the Amida, he should not begin the repetition until at least nine men are able to listen and respond, unless this would cause undue convenience, in which case the Hazan may begin if even five men will listen and respond.