The maximum number of atoms in a plane in ${\mathrm{PCI}}_5$is:

The maximum number of atoms in a plane in ${\mathrm{PCI}}_5$is 4.

Triangular bipyramidal shape .

The geometry of phosphorus pentachloride ${\mathrm{PCI}}_5$is trigonal bipyramidal.

It can fit a maximum of four atoms in the same plane (one phosphorus and three chlorine).