BT Megilla 5a records:
Mishnah: What is reckoned a large town? One which has in it ten idlers [בטלנין]... Gemara: A Tanna taught that the ten idlers attend the synagogue.
In fn. 21 of the Artscroll translation (JT) it explains that the בטלנין "idlers" are:
ten men who are supported by the community and refrain from other employment in order to insure the presence of a minyan in the synagogue during prayers (Rashi 3b, followed by Korban HaEidah; cf. Ran to Rif, Sdeh Yehoshua).
My question is: According to the view that they were there to insure the presence of a minyan, why 10?
If a man arrived at shul looking for a minyan, only 9 more would be needed.
So why pay 10? Are these 10 men praying on behalf of the kahal?