By January 1, 2020, the FAA says that your aircraft must be equipped with ADS-B Out to fly in the most controlled airspace. Federal Regulations 14 CFR 91.225 and 14 CFR 91.227 contain the details. The FAA estimates that between 100,000 and 160,000 general aviation aircraft may need to equip with ADS-B. Approximately 40,000 aircraft have been equipped with ADS-B Out avionics and are ready for the 2020 mandate. More than 30,000 aircraft have equipped with ADS-B In avionics.
Amstat and ADS-B Exchange (a global co-operative of ADS-B/mode-S/MLAT feeds) released a study that estimates that only 40% of US-based general aviation aircraft will meet the FAA mandate. They report that 39% of US-based business jets and 38% of US-based business turboprops are currently known to be compliant.
