Different views exist regarding the congregation's "Yeheh Shemeh Rabba" response during Kaddish. All agree that this response is associated with the number 28, but there is a disagreement as to how this association is to be expressed. The significance of this number in the context of the "Yeheh Shemeh Rabba" response is indicated by the Gemara, which speaks of the great rewards for reciting this response "Bechol Koho" – with all one's strength. The word "Koah" ("strength") in Gematria equals 28, and so we associate this response with the number 28.
The Abudarham (Spain, 14th century) maintained that the congregation should respond with 28 letters – "Yeheh Shemeh Rabba" through "Almaya." This response consists of seven words and 28 letters.
Rav Yishak Abuhab (Spain, 14th century), cited by the Bet Yosef, ruled that one should recite 28 words – from "Yeheh Shemeh Rabba" through "Da'amiran Be'alma."
The Bet Yosef writes that the first view is incorrect, as one may not end the response after the word "Almaya." He brings a Midrash warning of grave punishment for those who make a separation between this word and the next word, "Yitbarach." This point is emphasized also by the great Kabbalist Rav Yosef Gikatilla (Spain, 13th century), who writes of the importance of not separating between these two words. Accordingly, the Shulhan Aruch writes that those who end their response with the word "Almaya" act incorrectly. The Magen Abraham (Rav Abraham Gombiner, Poland, 1635-1682) understood the Shulhan Aruch to mean that one should continue his response through the word "Be'alma," though the Magen Abraham himself maintained that one should respond only through "Almaya."
Regardless, we generally follow the customs of the Arizal, who taught that one should respond through "Da'amiran Be'alma."
There are some who respond through "De'Kudsha Berich Hu," but this practice has no halachic basis and is incorrect. Those who are accustomed to doing so should discontinue this practice.
Among the Ashkenazim, many have the custom to respond only through "Almaya." This was the view of the Gaon of Vilna (1720-1797). He disputed the view that "Yitbarach" must be recited immediately after the word "Almaya," arguing that "Yitbarach" begins the next sentence. Nevertheless, some Ashkenazim add "Yitbarach" and conclude their response at that point. The Aruch Ha'shulhan (Rav Yechiel Michel Epstein, 1829-1908) observed that this was the prevalent practice among Lithuanian communities, despite the view of the Vilna Gaon.
It is worth adding further insight into the connection between Kaddish and the number 28 – which, as mentioned, is the Gematria of the word "Koah." Rashi, in his opening comments to the Humash, writes that the Torah begins with the story of the world's creation in order to establish that the entire world belongs to G-d, as it is He who created it, and He thus had the authority to give Eretz Yisrael to the Jewish People. If the gentiles challenge our rights to our homeland – which they of course do, especially in our day and age – we must know that Hashem created the world and distributed it as He saw fit, and He decided to give us the Land of Israel. Rashi cites the verse in Tehillim (111:6), "Koah Ma'asav Higid Le'amo, La'tet Lahem Nahalat Goyim" – "He told His nation the power of His deeds, in order to give them the nations' territory." The story of G-d's "power," the creation of the world, is our response when nations challenge our right to the territory that we seized from the people of Canaan. Not coincidentally, the opening verse of the Torah ("Bereshit Bara Elokim…") consists of seven words and 28 letters – just like "Yeheh Shemeh Rabba…" – and Rashi links this verse to the verse "Koah Ma'asav Higid Le'amo," which begins with the word "Koah." Through our response to Kaddish, then, we access Hashem's unlimited power, expressed most strikingly through the act of creation, and this gives us the ability to withstand any challenge from our adversaries.
It is worth noting in this context the famous Midrashic tradition that Moshe Rabbenu recited 515 prayers asking for the privilege of entering Eretz Yisrael (the numerical value of the word "Va'et'hanan"), and if he would have recited a 516th prayer, Hashem would have had to grant his request. The commentaries explain that this 516th prayer that Moshe would have recited was Kaddish, which begins with the words "Yitgadal Ve'yitkadash Shemeh Rabba." The first letters of these words (Yod, Vav, Shin, Resh) have a combined numerical value of 516 (10+6+300+200). This sheds additional light on the connection between Kaddish and Eretz Yisrael. When we recite and respond to Kaddish, we are asking that Hashem's Name should be glorified through our nation living in the land with the Bet Ha'mikdash under the reign of Mashiah, and this prayer has special power and significance.
In light of this association between "Yeheh Shemeh Rabba" and the number 28, it is critically important to enunciate each word. If one responds too quickly, he might likely fail to pronounce the word "Min" in the phrase ("Le'ela Min Kol Birchata"), and will thus recite fewer than 28 words. The Poskim write that one should respond "Yeheh Shemeh Rabba" slowly and carefully, ensuring to properly pronounce each word.
Summary: Our custom is to respond during Kaddish from "Yeheh Shemeh Rabba" through "Da'amiran Be'alma" – a total of 28 words. This number is very significant, and therefore one must ensure to respond slowly and properly pronounce all the words.