Thanks for your comments and explanation! Now I know…. it just didn’t sound right and I just followed Logos’s description.

Quote from Logos: “The Logos edition of Göttingen Septuagint contains 24 print volumes divided into 65 resources, with the text of the Septuagint and the critical apparatuses split into separate files for optimal use in your digital library. In addition to the Greek text and apparati, these volumes also contain a wealth of introductory and supplementary material on the text.”

The problem is that the Latin “apparatus” is fourth declension, so the Latin plural would be “apparatus”, with a long final syllable. This does not come over easily into English. I prefer “apparatuses” to “apparati”, because while neither form is correct Latin, at least the first follows the dominant pattern for pluralization in English.

However, the best solution may be to omit the plural altogether. If a text contains more than one apparatus (as the Gottingen Septuaginta does), you could say it has multiple levels of apparatus. In the sentence from Logos quoted above, they should perhaps simply say “the text and apparatus”.