Let's find out!
The number of militia ("patriot") groups is pretty well known, but their membership numbers can only be estimated. This seems like a solid estimate:
"The number of Patriot groups, including armed militias, skyrocketed following the election of President Obama in 2008 – rising 813 percent, from 149 groups in 2008 to an all-time high of 1,360 in 2012. The number fell to 1,096 in 2013."
Source: Southern Poverty Law Center
I'm going to guess that the average group has 100 members, about 40% are active. That would put an answer to your question at about 100,000 people, about 40,000 active.
https://www.quora.com/How-many-militia-members-are-there-in-the-US
Other estimates say militia membership ranges from 20-60,000. More recent estimates try to account for Trump's incitements, and claim there are more than 2,000 militia groups today--that Trump has doubled the "patriot" movement almost overnight.
It's reasonable to say that there are a thousand times as many potential domestic terrorists in America as there are potential Islamist terrorists in Germany. All those militia members, and the uncountable lone wolves, are pledged to take up arms against the government if it "goes too far", according to thoroughly twisted ideas of "too far".
Each one chomping at the bit to start shooting.