Halakhah su Salmi 28:78

Care should be taken not to take a head-count of people to verify whether there is a minyan,⁶A quorum of ten. because it is forbidden to take a head-count of Jews even for the purpose of a mitzvah, for it is written: "And Saul gathered the nation and counted them by means of lambs."⁷King Saul told each of them to take a lamb from the king’s flocks, then he proceeded to count the lambs, thereby avoiding the direct counting of the people. See I Samuel 15:4. It is customary to count them by reciting the verse: Hoshiah es amecha etc⁸Psalms 28:9. which contains ten words.

It is necessary that all ten [men] be in one place and the chazzan with them. If some are in one room and some in another room, they are not considered combined, though the door between them is open. Even if the majority are in the synagogue and the minority are in the vestibule in front of the synagogue, they are not drawn to the majority to be combined with them.⁹I.e., the minority on the outside remain a separate group and are not combined with the majority to make up a minyan. Even those standing on the threshhold, within the opening on its outer flank, that is—when the door is closed — that place is on the outside, though the door may now be open, nevertheless, it is considered outside. All this applies to combining ten [for a minyan], but, if there were ten and they are saying Kaddish, Barechu or Kedushah, then, whoever hears them may respond with them, even if many houses separate them [from the main minyan], because even an "iron curtain"¹⁰Lit. “iron partition.” cannot separate Israel from their Father in Heaven, provided that there was no excrement or idolatry present there.¹¹I.e., between the minyan and those responding.