According to the Substance Abuse and Mental Health Service's Treatment Episode Data Set, in 2015 in the US there were 213,001 admissions to treatment with marijuana reported as the primary substance of abuse out of the total 1,537,025 admissions to treatment in the US for those aged 12 and older for all substances that year. This is the lowest number of marijuana admissions and total treatment admissions in at least a decade: marijuana admissions peaked in 2009 at 373,338, and total admissions peaked in 2008 at 2,074,974.

Source

Substance Abuse and Mental Health Services Administration, Center for Behavioral Health Statistics and Quality. Treatment Episode Data Set (TEDS): 2005-2015. National Admissions to Substance Abuse Treatment Services. BHSIS Series S-91, HHS Publication No. (SMA) 17-5037. Rockville, MD: Substance Abuse and Mental Health Services Administration, 2017, Table 1.1A, p. 47.