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).

Please let radref know if you've got other questions, or you can visit the NYC public law library (map). FYI There are public law libraries across the country with knowledgeable reference librarians.