Given the importance of gangs' financial networks to their ongoing viability, that strikes me as an obscenely low number. Of course, Mexico should be doing a lot more than the US on the issue of Mexican gangs, and unfortunately Mexico (characteristically) does not make public any sort of information along those lines (at least, not as far as I know). On the US end, we're talking about around four businesses a year. Without even trying particularly hard, I could probably point you to four money-laundering businesses a year in one city just based on rumor (which of course is not a high enough standard for seizing assets, but certainly could point a professional investigator in the right direction).
On the plus side, kudos to the law's sponsor (Carl Levin) for squeezing the word "kingpin" into federal law.