If you're using the vocative case for a plural word like friends pluralized, would it be amici or amicis, or something different? Thanks!

2 answers

"In Latin, as in English, the vocative case, used in addressing a person, has the same form as the nominative, except that the vocative singular of -us nouns and adjectives of the second declension ends in -e (in -ius nouns, -ie becomes -î)." [from Latin for Americans: First Book (1968)]

The above is referring only to the singular forms of certain words. You are referring to the plural of amicus, right? The plurals are not affected by any exceptions; only some singular forms. Therefore, the vocative plural of amicus is amicî.
(That should be a macron over the second "i" in amici.)