This report, published in 2004, provides estimates of the number of individuals in the United States in 1999 who had mental health benefits as a part of their health insurance coverage, the subset of those individuals who had mental health benefits that met or exceeded a benchmark level of generosity, the number of individuals with parity in their mental health benefits, and the number of individuals potentially subject to state and federal mental health parity laws. 

From the report:

Mental health benefits are assessed in terms of the types of services covered (inpatient care, outpatient care, and prescription drugs), dollar limits (annual expenditures and lifetime expenditures), utilization limits (number of days of inpatient care and number of outpatient visits), and cost sharing (deductibles, co-insurance, and co-payments). The benchmark level of generosity is defined in this report in terms of the types of services covered (inpatient and outpatient care and prescription drug coverage) and utilization limits (20 inpatient days and 30 outpatient visits). Full mental health parity is defined as mental health benefits with the same covered services, dollar limits, utilization limits, and cost sharing as the plan's medical/surgical benefits.

Full report: http://mentalhealth.samhsa.gov/publications/allpubs/SMA04-3872/part1.asp#executivesummary

U.S. Dept. of Health and Human Services. Substance Abuse and Mental Health Services. (2004). National estimates of mental health insurance benefits. Maxfield, M., Achman, L and Cook, A.