A puppy has 32 teeth and when it is full grown, it will have 40. THis change starts after they are 6 months old. One day, we will notice our dog is missing a fang, which actually stand out from the rest, and then, we will start seeing newer and stronger replacements. There is a time in their lives in which they need to bite anything they find. This relieves the discomfort they feel.
When they are changing their teeth, all we need to do is to notice if both teeth are in their mouth. In other words, the older tooth (milk) and the new tooth shouldn't be there at the same time. If this happens, the milk tooth won't fall on its own and you need a veterinary to extract it. The dog needs to be anaesthetized to do this. If we were to leave both teeth, plaque will form between both teeth damaging our dog in the long run. This is very common in small race dogs, like the Yorkshire terrier.
This puppy is 7 months And as you can see from the picture, it couldn't replace its teeth completely. There is some visible plaque forming between both fangs.
This is the same dog after we extracted those teeth it couldn't replace on its own.