How common is it for somebody to get overdose?

Of what?: If you are asking about a prescribed medication, rarely if you take the medication as prescribed.

Overdose: If its a medication - then its rare if the medication is used as prescribed. If one is irresponsible and you take too much or take the prescribed dose and then mix it with another non-prescribed drug or alcohol then you could "overdose". People who overdose usually have not followed their md's instructions or have been irresponsible.