I have seen various estimates of the percentage of pistol brace owners who have complied with the new BATFE rule that deems braced pistols to be subject to prohibition or registration as Short Barreled Rifles (SBR) under the National Firearms Act (NFA).
The most defensible estimates seem to be around 2.5% compliance. The most generous estimates come in near 8% compliance, while other estimates come in as low as .6% (yes, 6/10ths of one percent).