People who go to stores are customers. People who go to doctors and hospitals are patients. People who go to accountants and lawyers are clients. But, people who go to libraries are patrons. And... Almost all the relative pronouns are singular. Words like everyone and everybody are because they refer to one person (one, body). The only relative pronouns that are always plural are both, few, many and several, which are easy to remember. Because there are only four.
