So, they've preempted this by stating they estimate there to be roughly 3 million braces in circulation.
Roughly 10% or so of what is estimated to be in circulation by honest assessment.
Where did they come up with the 3 million number? My guess is they're largely looking at an estimate of how many braces are utilized by police departments. Police departments, and other government offices, utilize braces for the same reason anyone does; pistols are not regulated across state lines. An SBR is. Departments, like everyone else, doesn't want the headache of dealing with the paperwork or the hassle associated with the SBR.
Departments, though, will have 80%+ compliance. I say 80% as just a rough guess -- some departments will just swap 16in barrels in and put a stock on it - others will put a stock on it and register it as an SBR, and some will simply not have good enough track of their own property to register them all. But, generally speaking, they'll comply nearly 100% of what they know they need to comply with -- one way or another, most probably registering them.
So when the ATF comes rolling in about how successful this was that they got almost 100% compliance -- the media will roll with it, Democrats will claim victory, and normies are so stupid they'll believe it like they do everything else the zombie tube tells them to believe.
So, yes, when the deadline has come and gone they will announce a miniscule number of braces have been registered. We will all laugh... until we see it portrayed as a huge success on TV and America believes it.
That's my take on what they've got planned. They threw out that 3 million number preemptively because they knew this was going to fail and they wanted to tout a win anyway to try to convince everyone that America will bend the knee and lick their boots.