There's about 1.2 million active duty and another roughly 800k reserve and guard members. 

There's over 17 million veterans, plus tens of millions of patriotic citizens. They own about 20X more guns than the military.

But...

There's about 12 million military age illegal immigrants who could be conscripted in exchange for citizenship. 

There's millions of left leaning citizens who could be drafted in a civil war. 

There's NATO and the UN who would also be brought in on the side of the leftists/globalists if there was an American civil war.