Do you mean the percent of controls to non-controls?There is no such law, but rest assured if you are selling a high percentage of controls, DEA will come knocking. An appropriate percent of controls versus over all volume is no more that 20-30 percent. Anything more than that and you will be considered a narc mill.
Like Doctor M said, there's not an official regulation on the number of controlled Rxs a pharmacy can fill. However, the DEA does have a cap on the amount of controlled substances a pharmacy can sell to another pharmacy without registering as a distributor (no more than 5% of annual controlled substance sales).