This is one of those very tricky area as you can see by the comments already made. I will throw a bit more confusion in:
First agreement with what Suck My Glock said. I can’t find any law that covers this unless you build or have an intent to sell when building. The other part is some magic number determined by ATF that is a threshold making you a manufacturer. That same threshold applies to being a dealer. There is no number is US Code or ATF rules.
Then I will ad something to what Crippledtrigger said. If you do mark a firearm as a manufacturer there ae requirements you must follow, the depth and size of font and it must include your name, city, state, and serial number. Here is the rub in my mind, if you mark it like that have you admitted you are a manufacturer? If you “manufacture” an SBR or an SBS you must mark it like that and pay a tax under NFA. I can’t find a law that says a firearm you manufactured for yourself must be marked if you sell it.
Add more confusion to the magic number thingy that makes you a manufacturer. When you build one SBS, SBR, or Suppressor, you submit a form 1 stating you are the manufacturer. There are 10s of thousands of those in the system already. Could the ATF go to court and say, see the magic number is 1 nobody has to build 2 of anything to be a manufacturer, look at this mountain of evidence.
This will likely require a test case and I can’t afford to be that person. I would rather destroy the “firearm”, the AR, AK, Glock, or 1911. It would be far less costly in the long run.