When people see themselves in the mirror, they look proper but the number and alphabets look opposite. Why?

It happens due to the property of the mirror termed as "Lateral Inversion".

It is a phenomenon in which when an object is placed in front of the mirror the right side of the object becomes left side of the image and the left side of the object becomes the right side of the image.

It also happens to us, if you pay attention while watching yourself in the mirror you will see that your right hand will be the left hand of the image and the left hand will be the right of the image.

Example- In the ambulance van, the word AMBULANCE is written in reverse order so that when the word is viewed through the rearview mirror (having convex mirror which provides a wide view of the field) of the vehicle then the driver could easily get to know about the ambulance and give way to it.