Is pure cow milk a pure substance or mixture?

Answer Yes, milk is regarded as a mixture and not a pure substance. To be more specific, milk is a colloidal mixture. In a colloidal mixture, particles are suspended freely in the solvent.

In milk, the particles consist of fatty acids, proteins, mineral ions etc. in a solvent of water (constituting most of the solvent part).

Colloidal mixtures exhibit a typical effect called the Tyndall Effect. You can demonstrate Tyndall Effect in your room itself. Take a jar containing a mixture of milk and water in an approximate ratio of 5:1. Place it in your room and darken the room by turning off all lights and putting the curtains. Apply a focus of torch light on the mouth of the jar. The shine produced as such is called the Tyndall Effect, which is exhibited only by colloidal mixtures like milk, BLOODY ETC.

It is a mixture of many, many different substances.

it is a mixture so,it is not a pure substance