I want to reply to this comment:
What's the percentage of people that are mental calculator prodigies? How do they spread over the different parts of the world?
There are two common ways someone can become a mental calculator via high iq or by savant ability. In exceptional cases someone may develop this ability while having only a average IQ but the level of dedication to such abilities is pathological interfering with normal development on the order of OCD.
Assuming the mental calculator abilities are the function of a savant ability. 62 out of every 10,000 are autistic and 10% of all autistic person have a savant ability finally half of all savants are autistic so multiply the number of autistic savants by two. Which gives you 12/10,000 as a maximum amount of mathematical savants.
About 1/1,000 people are geniuses with iq of 145 but i suspect it takes an Exceptionally Gifted person with an iq above 160 1/10,000 to have the capacity to become mental calculators.
So by my high estimations between 1 and 5 people out of 10,000 are capable of being mental calculators. Because savantism, autism and iq occur semi randomly (due as best we know to genetics) you should expect their to be an equal distribution among all populaces.