The secondary valency of Chromium in $\left[\mathrm{Cr}{\left(\mathrm{en}\right)}_3{\mathrm{Cl}}_3\right]$ is

Primary valency of a complex corresponds to the oxidation number of metal and the secondary valency corresponds the coordination number of the central metal ion of the coordination complex.

Oxidation number of Cr is +3, this implies that the primary valency of the complex is 3.

There are 2 bidentate ligands (en) and 2 mono-dentate ligands (Cl) that are attached to Cr atom. So, the coordination number of Cr will be 6. Hence, the secondary valency of the complex is 6