There are 322,772,000 people in the US. Of these, about 13 million are Legal Permanent Residents without citizenship. 11 million are illegal immigrants. The State Department estimates about 5 million US citizens living abroad.
So the number of US citizens is approximately 313 million. Approximately 20 million (accounting for naturalized expats) did not have US citizenship at birth. This leaves 293 million native-born US citizens.
The second requirement is to be over age 35. 52.8% of the US population is over age 35. We will take a risk (for lack of data) and extrapolate this to all native-born US citizens. This gives us 155 million native-born US citizens over age 35. This is likely an overestimate because the median age of immigrants to the US has been higher than that of the general population, so native-born US citizens would likely be younger than naturalized immigrants, on average.
The third requirement is that one must live in the US for 14 years. This is very vague and has never been challenged. Likely, some of our expatriates that we factored in have spent less than 14 years living in the US. Let’s take the statistically safe estimate and say 50%, taking away 2.5 million people.
So we have about 152 million people who fulfill the requirements to be President of the United States.