Of the 3,539,950 adults on probation in the US at the end of 2018, 26% (approximately 920,387 people) had a drug charge as their most serious offense.
Source
Danielle Kaeble and Mariel Alper, PhD. Probation and Parole in the United States, 2017-2018. Washington, DC: US Dept of Justice Bureau of Justice Statistics, August 2020. NCJ252072.