If you find this awkward, you can substitute the n with the prefix ne instead. I suspect that Dr. Elgin made this rule up for phonological variety anyway, and in fact forgets to follow it herself at points throughout her grammar book. I therefore don't feel too guilty about not following the rule myself. Or a speaker may substitute a noun with a pronoun in subsequent sentences, and since pronouns do differentiate for number, you can tell whether the original noun being referred to is singular or plural. Or, finally, it may be obvious from context.
Case is indicated with suffixes.
Other examples could be given of instances when a noun needs to take both the possessive and the instrumental case suffixes, the possessive and objective suffixes, etc. For plural indefinites, you can then add zh or n after that. For plural reflexives, you can then add zh or n after that. This can be a weird concept for English speakers to grasp, and will be covered with more explanatory detail later. They are never modified and they come directly after whatever they refer to.