There's no single answer to what percentage of university students major in law. The reason is that a law degree (Juris Doctor or J.D.) is a *professional* degree, not an undergraduate degree. Students typically complete a bachelor's degree in *any* subject before applying to law school. Therefore, calculating a percentage of undergraduate students majoring in "law" is meaningless.
To put it another way: You might find statistics on the number of people enrolling in law school each year, but that's a different metric than the percentage of undergraduates majoring in law (which is effectively zero).