I asked Dr. Aileen Buckley, Esri Cartographer for her advice. She wrote a blog post that explains when you should use singular and plural for legend items. I asked, ”Is there a cartographic convention or guideline for legend item labels being singular or plural? Do plural labels imply something different if there are both singular and plural entries?”
In summary;
"As a rule, legend times labels should be singular nouns."...
Also, ”Do plural labels imply something different?” Yes, they do. The most common interpretation of legend item labels that are plural is that there are multiple things of the same type in a single location."
Entry on the ArcGIS Blog.abuckley-esristaff
Legend item labels – singular or plural | ArcGIS Blog