Let's examine each of the given species to determine if the central atom's hybridization involves the dz2 orbital. Keep in mind that d orbital participation in hybridization typically occurs for central atoms from the third period onwards, as they have accessible d orbitals.

First, consider I3−. The central iodine atom has a steric number of 5 (2 bond pairs and 3 lone pairs), leading to a linear shape due to sp3d hybridization. This sp3d hybridization involves one d orbital, and conventionally, this is considered to be the dz2 orbital.

Next is PCl5. The central phosphorus atom has a steric number of 5 (5 bond pairs and 0 lone pairs), resulting in a trigonal bipyramidal geometry with sp3d hybridization. Similar to I3−, this hybridization includes one d orbital, often taken as the dz2.

For IF7, the central iodine atom has a steric number of 7 (7 bond pairs and 0 lone pairs), leading to a pentagonal bipyramidal geometry with sp3d3 hybridization. This hybridization involves three d orbitals, including the dz2 orbital along with dx2−y2 and one other d orbital (dxy or dxz or dyz).

Now consider NH4+. The central nitrogen atom is from the second period and has a steric number of 4 (4 bond pairs and 0 lone pairs), resulting in a tetrahedral geometry with sp3 hybridization. Second-period elements typically do not involve d orbitals in hybridization due to their high energy. Therefore, dz2 does not participate here.

For CH4, the central carbon atom is also from the second period and has a steric number of 4 (4 bond pairs and 0 lone pairs), leading to a tetrahedral geometry with sp3 hybridization. Again, no d orbital, including dz2, participates in this hybridization.

In CO2, the central carbon atom has a steric number of 2 (2 double bonds and 0 lone pairs), resulting in a linear geometry with sp hybridization. No d orbitals are involved in sp hybridization, especially for a second-period element like carbon.

Finally, consider XeF4. The central xenon atom is from the fifth period and has a steric number of 6 (4 bond pairs and 2 lone pairs), resulting in a square planar geometry with sp3d2 hybridization. This hybridization involves two d orbitals, which are typically considered to be dz2 and dx2−y2. Thus, dz2 participates in the hybridization of XeF4.

In summary, the species where the dz2 orbital participates in hybridization are I3−, PCl5, IF7, and XeF4. Therefore, there are 4 such species.