The answer is: it depends on how you measure it and how you define libertarian. The overwhelming body of literature, however, using a variety of different methods and different definitions, suggests that libertarians comprise about 10-20% of the population, but may range from 7-22%.
Furthermore, if one imposes the same level of ideological consistency on liberals, conservatives, and communitarians/populists that many do on libertarians, these groups too comprise similar shares of the population.
In this post I provide a brief overview of different methods academics have used to identify libertarians and what they found. Most methods start from the premise that libertarians are economically conservative and socially liberal. Despite this, different studies find fairly different results. What accounts for the difference?