The answer is Chicago-based Arthur Andersen, with more than 3,600 lawyers. Except Andersen isn't a law firm. It's an accounting firm that offers all kinds of advice to businesses. The lawyers at Arthur Andersen and other accounting firms don't really practice law, we're told. They offer clients advice on things such as taxes.
Goodness knows companies can use such advice.
Federal tax law is so complex that I doubt if anyone really understands all of it. But not in this country. Just because the ABA says no doesn't mean it won't happen. The real decisions about the practice of law are made by state bar associations. Still, the ABA stance tends to carry huge weight among state bar regulators and legislatures.