If your patron is only looking for a number, the almanac or atlas would have it, but these may be out of date if relying on older census information. Even the simplest question can have conflicting answers in different reference sources, e.g. for issues on world population. Any information that changes often is seldom accurate in printed sources. Also consider how difficult it is to determine accuracy and currency for Internet resources. If you live in a major city, population information will be easier to find, even in entries for the state in some encyclopedias and in the almanacs.
Road atlases usually show city populations for most small towns.
Dictionaries often have surprising information in appendices. The local Chamber of Commerce or tourist board often have demographic information available. Keeping track Study the form that your library uses to keep track of reference questions. ORE on the Web, Ohio Library Council, 2000. See Copyright-Credits page for information regarding design, source of materials, and for use of the site.