It is most appropriate that the tag names of countable nouns should be in plural forms, but there are still quite a lot of tag names which should be in plural forms but they use single forms, for example the indefinite-integral and definite-integral tags (more examples can be found in e.g. Renaming tags to plurals), compare with the improper-integrals tag, this should be a great joke of the Q&A sites industry as it for example appears the double standard of the plurality of tag names.
To solve these problems, for the tags which have very few questions, it is practical to manually retag each questions of the tags, but it is not practical for the tags which have many questions. Another approaches are that first create the plural form tags for some questions, and make the applications of the single form tags merge into the plural form tags, but I am afraid for example there are too few users which can fulfil the minimum requirements for the voting actions of handing the applications so that the applications should take too long time or even impossible to become successful. So the best approaches should be by renaming of those tags. But the tags renamings can only be done by our diamond moderators with just taking some simple processes. But even they are so simple, most diamond moderators are still often not willing to do so and make me disappointed.