Q. If a document references only one figure, should it be labeled “Figure 1” or assigned no number?
A. A lone figure, even if it is referred to in the text, can usually remain unnumbered:
All but one of the posters relied on a conventional list; the outlier used an infographic (see figure).
The corresponding figure caption would begin with “Figure” or “Fig.” This approach is recommended by the AMA Manual of Style (11th ed., 4.2.7.1) and Scientific Style and Format (8th ed., 30.2.1)—though the word “figure” would be capitalized in direct references in both of those styles.
But assigning a number to such a figure would be appropriate in at least three scenarios: (1) the figure is the only one in a chapter in a book featuring numbered figures in other chapters; (2) the figure is the only one in an article in a journal whose house style requires assigning numbers for all figures for consistency across articles; and (3) the figure occurs in a context that also includes more than one numbered table (e.g., fig. 1 and tables 1 and 2).
CMOS, which is a general reference, allows for any of those approaches. But note that all figures, whether they will be numbered in the published version or not, should carry a working number in the manuscript (see CMOS 3.13).