This is a rare teeth condition called Hyperdontia . When a person has 20 more teeth on top of the average 32 (so 52 or more total) have Hyperdontia . These teeth usually grow on the top of the mouth, but can occur at the bottom under the tongue also. People with this condition usually have it corrected when they are younger or when their permanent teeth start growing in. Most people who have Hyperdontia have another set of teeth behind their main teeth. The picture above shows a person with 76 teeth in his upper mouth area. This is a very extreme case of the condition.
If you have normal teeth, you should feel very blessed. I feel sorry for sufferers of this condition. Having teeth like this must be so annoying. Imagine chewing food and feeling the teeth on your tongue, that must be disturbing. Nightmare sessions at the dentist for sure :O.