Are the cases in Latin always six?

Question

In a book about linguistics I've read this sentence:

Each word has up to six different such 'cases', and each case has distinct endings for singular and plural.

Now I'm pretty sure that when I studied latin for 1 year during middle school and 5 years during high school, the latin cases were always (I mean, in every declension, and more in general in every discussion) referred to as 6 in number, whether some of them have the same ending or not.

But the sentence above gives me 1% chance that I might be wrong and that you count n cases with the same ending as 1. Which also means that given a noun I might find plural and singular have different number of cases. Which pulls that 1% down to 1‰.

But why not asking?

Furthermore, I've not studied the history of the latin language, so I have no idea whether there where more cases in earlier times, which would make me think that there could have been a time where some words had, say, 6 cases, while others had 7.

Answer 1

Generally, most nouns and adjectives have six cases (nominative, genitive, dative, accusative, ablative, and vocative). However, this isn't always the case.

Some words are defective, where they appear to lack a case, and not just because of lack of attestation. These include very popular nouns like fas, nefas, nihil, and opus. You can see a fuller list of them in Allen and Greenough.

With other words, however, there's a seventh case: the locative. Words which have a locative case are usually where you can be, so you can find humi, domi, and ruri for humus, domus, and rus. You'll also see it with cities, so Romae or Carthagini are locative forms.

Answer 2

Furthermore, outside of non-neuter words in the second declension the nominative and vocative aren't formally distinct, and for neuter nouns of any declension, the nominative and accusative aren't formally distinct either.

Additionally, in o-stems (i.e. the second declension) and many i-stems (a subset of the third declension) the dative and ablative aren't formally distinct, with the singular for both ending in -ō (for the o-stems) or -ī (i-stems; analogy with other words in the third declension introduced -e for the ablative for many i-stems as well, though), and the plural in -īs (o-stems) or -ibus (i-stems)—the dative and ablative plural are never distinct in any paradigm, but the singular usually is.

Obviously the vocative resp. dative/ablative are all still felt to be distinct cases by speakers even if they're formally indistinguishable, as e.g. agreeing adjectives in different paradigms demonstrate, but this could be what your source was referring to.