In this case the mean IQ is 100.
The standard deviation is 15 points. The geniuses have IQ of 145 or above.
This means that it is equal to three standard deviations. This comprises of people with IQ of 145 or above (100+45) and also people with IQ of 55 and less than that (100-45).
In a normally distributed population, the percentages of people, which come within +3 standard deviations, make up 99.7%.
0.15% or half of the 3% of people remaining will have IQs 145 or above than that.
In the total population of 300 million people, 0.15% makes 450,000 people.
So, there may be 450,000 people who are expected to be geniuses.