Must one have a Minyan for Kerias Hatorah and how many people must be paying attention?
One may not read from the Torah [with its blessings] if there are less than ten adult men above the age of Mitzvos who are present. [Accordingly, one is required to have a Minyan to be able to perform Keiras Hatorah, just as is the rule regarding any Davar Shebekedusha, such as for Kaddish and Chazaras Hashatz.]
“Asara Mitzaytin”-Ten people who are paying attention to the reading: It does not suffice for there simply to be ten men present in the room for one to be able to perform Kerias Hatora. In addition, there must be ten men who are paying attention to the reading in order for the Minyan to be valid. This is known as “Asara Mitzaytin.” [Thus, if one of the ten men is on the phone, or studying Torah, or involved in any other subject, and will not be stopping to hear Kerias Hatorah, then he cannot be counted as part of the Minyan of ten. Accordingly, when making a Minyan for less than ten people who did not hear Kerias Hatorah, one must verify that those people who are completing the Minyan will concentrate and pay attention to the reading, otherwise it is invalid and the blessings are said in vain.]
 Michaber 143:1; Rambam Tefila 2; Mishneh Megillah 23b “And one may not read the Torah…with less than ten”; Ketzos Hashulchan 25:14; See Admur 55:2
 Implication of Michaber 146:2 “If there are ten people listening to Kerias Hatorah”; Implication of M”A 146:5 in name of Terumos Hadeshen [that only when one is reciting Shnayim Mikra there is a leniency that we don’t need Asara Mitzaytin]; Semak [Amudei Goleh] 155 “If there are ten people which place their ear to the Torah”; Biur Halacha 146:2 “Vilikros Shnayim Mikra” based on Rif, Bahag, Kol Bo, Hagahos Maimanis, and other Poskim that by Kerias Hatorah there must be ten listeners; Piskeiy Teshuvos 143:1; So rule regarding a Davar Shebekedusha that one who is sleeping, or talking, is as if he is not there: 2nd opinion in Admur 55:8 regarding sleeping; Admur 56:4 “If there is less than nine people listening to the Chazan, then he should not say Kaddish at all, as every Davar Shebekedusha may not be said with less than nine listeners and one reader” ; Beis Yosef 125; Rosh 4:19
Other opinions: Some Poskim rule that it is permitted for a member of the Minyan to recite Shnayim Mikra during the reading, and he can still count for the ten listeners. [M”A 146:5, in name of Terumas Hadeshen, based on Or Zarua] The Biur Halacha ibid negates in length this opinion, and states that Kerias Hatorah is more severe than Davening, and thus if there are not ten people actually paying attention, the Minyan is invalid, and it is a blessing in vain to go forward with kerias Hatorah in such a state