Q) What is the minimum number of people that there needs to be who have finished the amidah – in order to begin chazoras hashatz)
A) The Shulchan Aruch[1] (Orach Chayim Siman 55) tells us that if an individual is davening [the Amidah], or is present during a Tefilla but sleeping – he can still be counted towards the minyan [i.e. to enable saying Kedusha]. The Mishna Berura[2] says that this leniency applies even concerning several people (i.e. who are currently davening or are sleeping) as long as the majority of the minyan are awake and listening to chazoras hashatz. (The Mishna Berura then brings another opinion who is stringent regarding more than one person sleeping.)
The Shulchan Aruch (Orach Chayim Siman 124[3]) rules that if there are not nine people listening to chazoras hashatz – it is [dangerously] close to chazoras hashatz being considered brochos l’vatolo.
Many commentaries ask that there appears to be a contradiction between the Shulchan Aruch in Siman 55 and the Shulchan Aruch in Siman 124: Surely the individual who is currently davening or sleeping (in Siman 55) is no better than the individual who is not listening (discussed in Siman 124), and therefore how can the Shulchan Aruch in Siman 55 validate chazoras hashatz where one of the members of the minyan is davening or sleeping, if he describes a seemingly similar situation in Siman 124 as “dangerously close to being brochos l’vatolo”?
The Graz resolves these two seemingly contradictory halochos as discussing different situations: Siman 55 is discussing reciting Kaddish, or chazoras hashatatz in a situation where the shatz has not previously davened the amidoh with the tzibbur – and consequently, there is no danger of “brochos l’vatolo” because even if there would be no minyan this tefilla is being “used” as his obligatory tefilla. In this situation even an individual who is sleeping can be counted towards a minyan.
However, in Siman 124 the Shulchan Aruch is discussing regular chazoras hashatz (i.e. where the shatz has previously davened the amidoh with the tzibbur) – in such a situation there must be nine people listening; otherwise it is considered “dangerously close to brochos l’vatolo”. Consequently, someone who is davening or sleeping (and obviously not listening to the shatz) does not count towards a minyan.
From the Bi’ur Halocho[4] it appears clear that the leniency of Siman 55 applies also to chazoras hashatz in a regular situation (and not just to Kaddish, or a specific situation of chazoras hashatz where the shatz has not previously davened as explained by the Graz). Rav Chaim Kanievsky[5] also understands the permissibility of Siman 55 to include a regular chazoras hashatz as well.
It would seem that the Mishna Berura himself learns that these two halochos of the Shulchan Aruch (Siman 55 and Siman 124) do in fact contradict each other, and the minhag is in line with the lenient opinion in Siman 55[6].
Based on this, it would certainly be acceptable to rely on the Shulchan Aruch in Siman 55, and include those people who are still davening in the minyan for chazoras hashatz and/or Kedusha – as long as they are a minority of the minyan (since according to the Mishna Berura and other Poskim, the leniency applies not only to one individual but even to several people).
Conclusion: When waiting for a minyan in order to begin chazoras hashatz it is certainly ideal to wait until there are nine men who have finished davening and will be able to listen to the shatz. In extenuating circumstances or when people are pressed for time, it suffices to have a minimum of five people (and the shatz) who have finished the Amidoh and to include up to four people who are still davening.
[1] או”ח נ”ה ו’
[2] שם ס”ק ל”ב
[3] או”ח קכ”ד ד’
[4] ס’ נ”ה ד”ה או שהוא
[5] בהערות לס’ אשי ישראל מהדורא בתרא תשובה כ”ב
[6] ע’ מג”א ס’ נ”ה בשם המהרי”ל, וע’ מ”ב קכ”ד ס”ק י”ט שמביא דעת המג”א