If you are near an academic or large research library it is best to try this subject in some major databases.

Lexis Academic gets 30 hits in current 2 years for the search: "civilian review boards". Factiva (a better source of news articles than Lexis, thinks a social science librarian colleague here at U of MD) gets 66 in the same 2 years. I would recommend browsing all 30 in Lexis, because trying to limit with words such as "overview" or "survey" or "statistics", etc, risks losing a good article that omits those words. In Factiva, it makes more sense to try some extra words. "Civilian review boards" and survey gets 5 hits, including "Study says about 8 percent of U.S. police brutality complaints justify discipline" By MICHAEL J. SNIFFEN Associated Press Writer 404 words. Associated Press Newswires 26 June 2006 01:00 AM, which refers to a Justice Department survey of large state and local forces. In Dissertation Abstracts, the search: "civilian review" and "police departments" gets 3 dissertations, but all of them seem to be case studies of a single or only a few departments. Although I didn't see very promising initial results, it would be wise to try other searches such as "police departments" and Statistic* and "review board", etc.

The search "Civilian review boards" survey in USA.Gov gets 47 hits, but that DOJ report mentioned by Associated Press above is too recent to be one of them. In the Justice Department site search, I get only about 23 hits for the search "Civilian review boards", and the most recent appears to be Protecting Civil Rights: A Leadership Guide for State, Local, and Tribal Law Enforcement, Prepared by the International Association of Chiefs of Police September 2006. It has a lot of discussion about civilian review boards, but I don’t see statistics in it, from a fairly quick CTRL-F for the phrase civilian review. The search: statistics "civilian review" "police departments" gets 41 hits, but does not look too promising for statistics on numbers of review boards

A Google search: "civilian review boards" "monitor police conduct" gets 9 hits, including New York Times, Wednesday, June 4, 2008, that has an article "Report Details Variations in Police Review Boards" By MARIA NEWMAN Published: January 10, 1993, which says:
"As New York City prepares to institute an all-civilian review board to monitor complaints against the police, a report released yesterday by the New York Civil Liberties Union found that similar agencies throughout the country operate with significant barriers. The group's report also found that despite budgetary, legal and political constraints, more and more cities are establishing separate civilian agencies to monitor police conduct. Of the nation's 50 largest cities, 33 now have some form of civilian review procedures, and just over half were established in the last two and a half years, the report said."

National Criminal Justice Reference Service links to seemingly hundreds of hits, with the search: statistics "civilian review" "police departments, but recent ones don’t seem to pop out easily., and the few older ones I checked did not mention review boards.

In short, I think this will require many hours of research to get a definitive figure. I suspect Google searches may be faster for getting you into the general ballpark, especially if you use site:edu or site:gov to limit to more "official" sites. You can then go into those sites for more details.