According to Werner's Coordination Theory:

1. Primary Valence: Ionizable valence (oxidation state). For $Co$, it is $+3$. Cl⁻ ions satisfy this.
2. Secondary Valence: Non-ionizable valence (Coordination Number). It represents the number of ligand donor atoms bonded to the metal.

• In $[Co(N{H}_3{)}_6]C{l}_3$, the species inside the square brackets constitute the coordination sphere.
• Six nitrogen atoms from six $N{H}_3$ molecules are directly coordinated to the Cobalt center.
• Therefore, the coordination number is 6.