Mishnayos Bava Basra Perek 6 Mishnah 8

There is the case of one who sells a plot of land to another in order for him to construct for himself an underground catacomb, and similarly the case of a contractor who receives a plot of land from another under a commission to construct for him a catacomb. If the size of the catacomb was not specified, then he should make the inside of each burial chamber four cubits wide by six cubits long and open up into the chamber, by digging into its walls, eight burial niches [kukhin] in which the coffins will rest. Three niches should be opened up from the wall here, along the length of the chamber, and three from there, along the other side, and two niches from the wall facing the entrance. And these niches should be formed so that their length is four cubits and their height is seven handbreadths, and their width is six handbreadths. Rabbi Shimon says: He should construct the inside of each burial chamber six cubits wide by eight cubits long and open up into the chamber, by digging into its walls, thirteen burial niches. Four niches should be opened up from the wall here, along the length of the chamber, and four from there, along the other side, and three niches from the wall facing the entrance, and one from the right of the entrance and another one from the left. The mishna describes the general structure of the catacomb: And he must fashion a courtyard at the entrance of the burial chamber that should be six cubits by six cubits, which is equal to the combined length of the bier of the deceased and those who bury him, to ensure adequate room for the burial to take place. And he should open up two burial chambers into the courtyard, one from here and one from there, i.e., on opposite sides of the courtyard. Rabbi Shimon says: He should open up four burial chambers, one on each of the courtyard’s four sides. Rabban Shimon ben Gamliel says: Everything is dependent on the nature of the bedrock. If the bedrock is hard and strong it will be able to accommodate more niches, which will be more closely packed together, with less bedrock between them. If the bedrock is softer, fewer and more sparsely spaced niches should be formed.