May more than one person say Kaddish if there is exactly a Minyan or ten people present?
It is permitted for many people to say Kaddish together even if there will not remain a Minyan to answer Amen. This allowance applies even if the entire Minyan is reciting Kaddish and not even one person remains to answer Amen. Nevertheless, it is best to have at least two persons, or at the very least one person, remaining who will answer Amen to all the other members of the Minyan saying Kaddish. 
 Pesach Dvir 125b and Bnei Yehuda 3 and 22; Rav Poalim 2/14 and Ben Ish Chaiy Vayechi 16 in name of Rav Eliyahu Mani and Poskim ibid; Mishnas Sachir 1/11 in name of Poskim ibid; Kaf Hachaim 55/31
The reason: As the angels which hear the Kaddish answer Amen. [ibid]
 Mishnas Sachir 1/11 and Minchas Yitzchak 4/30 write one is to have at least two people remaining [based on Machatzis Hashekel 6 that one is to have two people answering Amen for a blessing; See also Shach C.M. 382 4]; Ben Ish Chaiy and Rav Poalim ibid write that there should be at least one person remaining who answers Amen, although he justifies even if no one remains to answer Amen.