None. The Brillouin zone is in reciprocal space, the atoms are in real space. The analogue of the Brillouin zone in real space is called the Wigner-Seitz cell. The number of atoms in a Wigner-Seitz cell depends on the kind of lattice, e.g. the Wigner-Seitz cell of a simple cubic lattice contains a single atom.

(left) Wigner-Seitz cell of a body-centered cubic lattice, image taken from Ashcroft & Mermin, (right) Brillouin zone of the same lattice, image taken from Kittel.

Quanti atomi ci sono in una zona di Brillouin?

In un cristallo? Tutti.

Le zone di Brillouin sono nello spazio di quantità di moto, non nello spazio di coordinate cartesiane.