For example, according to the Southern Poverty Law Center (not a group with an incentive to deflate numbers), at its peak the National Alliance had 1,200 members. All together, there are a few thousand active Neo-Nazis in the United States.
In contrast, let's take 2 countries where advocating Nazi ideology is illegal: Austria and Germany.
In Austria, the Freedom Party, founded by a former SS officer, has 50,000 members, 13 seats in the Upper House (similar to the Senate in the US) and 38 seats in the lower house as well as 4 in the European Parliament.
In Germany, the NPD received over 600,000 votes in the most recent election and now has a seat in the European Parliament.