Perhaps millions - perhaps 10. The odds of finding a true doubled die coin is dependent upon two things - the total mintage for a given coin and how many coins were struck with the doubled die. Both of these numbers can vary greatly. In some cases a given die may be used to strike as few as a few hundred coins. In others it may strike as many as 2 or 3 million. If it is an error die then it also depends on whether or not the Mint workers find the error before the die's life is expended. So there is no way to answer your question.
