Nouns like amicus (friend) fall into the second declension noun category. There are also nouns that end in icus that have to do with professions: medicinus (doctor) and mathematicus (mathematician) are both examples. Now, the plural endings for second declension nouns

N : ī

G : ōrum

D : īs

Acc : ōs

Ab : īs

The endings for the Latin word for sleep: (I’ve thrown in the locative and vocative for extra practice):

N : somnī

G : somnōrum

D : somnīs

Ac : somnōs

Ab : somnīs

L : somnīs

V : somnī