The ADL estimates the number of Neo-Nazis in the entire USin 2017 to be between 2500 and 5000 total members:
Neo-Nazis comprise one of the most visible and public elements of the white supremacist movement, as well as one of the most detested. Pretty sure the ADL has the best information on this, and while tracking these groups they are noting that they have been in steady decline, even with Trump in office.
The neo-Nazi movement arguably reached its peak in the 1990s, thanks to the National Alliance and its founder, former Rockwell disciple William Pierce. The National Alliance was the largest neo-Nazi group in the United States, numbering some 1,500 members at its height. The National Alliance even purchased a white power music company, Resistance Records, to increase its reach and bring in more cash.
However, the story of the neo-Nazi movement in the 21st century has largely been one of declining numbers and importance, at least in terms of organized groups. Pierce died suddenly in 2002 and his successors virtually destroyed the National Alliance, causing factionalization, mass defections, and the collapse of Resistance Records.
This decline was echoed by Aryan Nations, the second most important neo-Nazi group. Defections and infighting began even before leader Richard Butler's death in 2004 and continued long after. Today there are a variety of competing Aryan Nations factions, with Dennis McGiffen's Illinois-based Aryan Nations Sadistic Souls competing with Morris Gullet's Louisiana-based Church of Jesus Christ Christian for the top of the heap. Nevertheless, Aryan Nations today is much smaller than its 1990s equivalent.
More or less by default, the largest neo-Nazi group in the United States is the Detroit-based National Socialist Movement (NSM), with around 350 members in the entire country.
Other neo-Nazi groups are all much, much smaller, with membership in the dozens. They tend to be either small, isolated descendants of the American Nazi Party or short-lived new groups that form largely from splits and defections in other organizations and which tend to cannibalize similar groups for members until they too break apart.
Overall, then, the organized portion of the neo-Nazi movement has for some years been in relatively poor health. ADL estimates the total number of neo-Nazi's in the United States to be between 2,500 and 5,000.