Q. Can one person who is being paid for saying kadish, do so for other people at the same time and does he have to inform them?

A. Many Poskim rule that one may accept payment to recite kadish for a number of deceased (Even Ya’akov 50, Be’er Moshe 4: 97, see also Dovev Meishorim 2: 15, Minchas Yitzchok 3: 144, et al.).
However, Igrois Moshe (Y.D. 1: 254) maintains that one should recite at least one kadish a day for each of the niftorim. Therefore, the number of departed he can accept to recite kadish for them is limited to the number of kadeishim he can recite each day. Nevertheless, he may also attend a second minyan or include the kadish after learning in the list (ibid.). He should also keep a roster of the names of the deceased and mention them before each particular kadish.

Igrois Moshe (ibid.) also opines that he has to inform each of his clients that he is only reciting one kadish per day and that in Halacha it suffices. Unless, it is already well known fact at that venue, that this is the normal procedure of the people hired to recite kadish. (See also Kol Bo p. 376, Mourning in Halacha p. 363).
Horav Shlomo Miller’s Shlit”a opinion is similar to the Igrois Moshe.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a