As of 2016, the estimated population of the United States was 3.231 million people, according to the US Census Bureau. It is hard to get a specific number.
The Census Bureau has a cool tool that estimates the exact number of people in the US at any given time, according to general birth and death rates, and you can see how quickly the number changes. Find it at https://www.census.gov/popclock/. The “population clock” also shows the number of people in the world.
The US has the third largest population by number of people compared to other countries, with China being the most populous (1.379 billion) and India second (with 1.281 billion). The US is just one third of those countries but comes in third.