When someone dies and has no one male descendants to say kaddish for them, a person with no parents is often paid by the family to say Kaddish. Can the person who says Kaddish take money from more than one person to say Kaddish or must he say exclusively for only one person? The Tzitz Eliezer (Even Yaakov 50) says that one Kaddish can be a z'chus for many people and therefore he may take money from more than one person to say Kaddish. He does not even have to tell the person hiring him that he is saying for other people as well. He brings proof from various early poskim including the Rema and the Shu"t Maharanach.
He also brings the Kneses Yechezkel that says similarly if a person davens in a Shul where only person at a time says Kaddish, a person who is saying Kaddish for both his father and his mother does not get to say an additional Kaddish because he is saying for two people since his one Kaddish can go for both his parents.
Rav Moshe Feinstein does not agree with this psak and says in the the Igros Moshe that if he takes money from more than one person, he should make sure to say at least one kaddish per day for each person he took money from and write down which kaddish is for which person.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.