Laws of "Devarim Shebekedusha" - Part 1
The Kaddish Prayer, as well as Kedusha and Barchu, are all considered “Devarim Shebekedusha”(“Holy matters”), and therefore cannot be recited unless there is a minyan of ten men present. This halacha is learned from the verse, “I (G-d) shall be sanctified amidst the Children of Israel (Vayikra 23:32).” By connecting this verse with the verse, “Separate yourselves from amid this assembly.” (in Bamidbar 16:21) we learn that an official assembly of men which constitutes “a congregation” is required for the recital of Kaddish, and that the minimum number of people for this is ten.
Other Devarim Shebekedushainclude Kri’at Shema, the public Torah readings (on Mondays, Thursdays and Shabbat for example), the Haftarah (a section from the prophets read our aloud in the Synagogue on Shabbat after the Torah reading); Nesi’at Kapaim (lit. “raised hands”, referring to the special blessings by the Kohanim recited with their hands raised and “The Thirteen Attributes of Divine Mercy” (beginning with Hashem, Hashem….).
The conclusion of the halachic authorities is that this rule is of rabbinical origin, as is the entire concept and strusture of daily, organized prayer. In the event that a Davar Shebekedusha is said without ten men present, the people present do not fulfill their obligation, and it is therefore necessary to say it again in the presence of ten (Birkat Habayit). However, if there was thought to be ten men present — for example, in a case where one person left without the others realizing it — Kaddish or Kedusha and the like does not need to be said again as it is considered as though their obligation is fulfilled. This applies when at least six people remained — with less than six people even “after the fact” their obligation is not fulfilled (Piskei Teshuvot).
When reciting a Davar Shebekedusha it should be said loud enough for ten people to hear. Not less than seven Kaddeshim should be recited each day, as is hinted to in the verse, “Seven times a day I have praised You (Tehillim 119:164).” However, according to the Arizal twelve Kaddeshim should be said each day: Six during Shacharit, two during Minchah and four in the Ma’ariv service (Kaf HaChaim). Kedusha is said twice a day, in Shacharit and Mincha, during the second blessing of the Shemoneh Esrei when it is repeated out loud for the congregation. Barchu is also said twice a day: in Shacharit and in Ma’ariv, just before the blessings of the Shema.