If I have someone's license plate number, can I look up who owns the car?

An ordinary person cannot, I think so. In my opinion, states only store this information for law enforcement on a need-to-know basis. A police officer can do this, but it is better to have a compelling reason, such as part of an investigation or stopping traffic. Many provided services to others, such as friends, family, or city officials. Perhaps there is a way to find this information on the Internet?