I talked with a law library colleague about this question. The answer is not as cut and dried as the answer "70" above makes it seem. The LexisNexis results are basically how many cases cited 18 USC 2101, not how many people were charged under 18USC2101. Additionally, Lexis results have some duplication as the same case may have been tried at several different levels (all the way up to the Supreme Court) and cited the same law each time. This is a subtle but critical difference. My law library colleague said there's no way to really do that search directly using the current tools available (LexisNexis and WestLaw). Looking at each case in depth is really the only way to get that number. I've attached the searches for 18USC2101 (rioting) and 18USC2102 (definitions).
Legal and medical questions pose a real hazard for librarians, because we have to avoid "Unauthorized practice of law" in most states. But we can suggest places for folks to look, such as the state law and other Massachusetts sites noted in the earlier answers to this CORI question. Though they are no doubt heavily burdened by the present economic situation, it still might be worth looking into the Office of Labor and Workforce Development to find websites or local offices that could help advise you how to deal with these records.