Too Many People Saying Kaddish
When too many people recite kaddish at once, the result is often a symphony of kaddishes that are not well coordinated, and therefore, not being recited as they should be. The kaddish must be recited by all the mourners, simultaneously, word-for-word. What often happens is that some of those reciting kaddish will complete their segment -- which requires a response -- before others. Additionally, in such situations, responding to one person’s kaddish might leave one unable to respond to another person’s kaddish.
How should one to conduct oneself with regard to responding to kaddish when two or more people are reciting kaddish at different speeds?
According to the Mishna Berura, if everyone reciting kaddish is expected to conclude their segment within three seconds of each other, then one may respond to either the first or last kaddish to reach the end of the segment. If the gap between the kaddishes is more than three seconds, each kaddish should be responded to separately. It seems that if things are completely uncoordinated, then one should merely respond to the first kaddish.
There is also an opinion that one should wait for everyone who is saying kaddish to reach the end of their segment, saving one's response for when the last of these kaddishes is completed. Another approach is to concentrate exclusively on the kaddish that is being recited in closest proximity to oneself and simply respond to that one. Alternatively, one can respond to every kaddish that one hears and answer “amen” (or other response) to them all. This is true even if one would be forced to answer multiple “amens” one after the other, as long as one is able to properly concentrate on exactly what one is responding to.
Due to the confusion and disorder that can ensue when people recite kaddish at different speeds, those reciting kaddish should make an effort to recite it together in unison. Ideally, those reciting kaddish should do so in a centralized location, such as in front of the aron kodesh or at the bima, as is the custom in many congregations. In any event, one who recites kaddish must be sure to recite it loud enough for others to hear it and to respond accordingly. One who is not careful to do so is as if he did not recite kaddish at all. There is an opinion that it is best to disperse the mourners throughout the congregation. This is because by doing so it allows people to focus on a single kaddish that is being recited in their proximity rather than having to try and follow a kaddish that is being recited by all the mourners at once.
It is interesting to note that the original custom in Ashkenazi communities was for only one person to recite kaddish at a time. In such congregations, each kaddish would be delegated to a different mourner based on a halachic criteria of precedence. Although normative Ashkenazi practice is to allow all mourners to recite the kaddish together in unison, the practice of only one person reciting kaddish at a time still continues to this day in some communities. It is permitted to recite kaddish for a number of different people at once.
 Kitzur Shulchan Aruch Mekor Chaim 12:6.
 Mishna Berura 55:4.
 Shaarei Teshuva 56:3.
 Eishel Avraham 124:6.
 Shulchan Hatahor 132:4; Mishne Sachir 1:11.
 Mishna Berura 6:28, 124:25.
 Tzitz Eliezer 9:16; Teshuvot V'hanhagot 1:103, 2:42.
 B’tzel Hachachma 5:135.
 Mishne Sachir 1:11.
 See Biur Halacha 132. See also Tzitz Eliezer 9:16:4.
 Gesher Hachaim30:8:8.