How i remember is -'A" is feminine so use 'a' ending. English vowels endings e,i,o,u are 'e' and words ending in consonants use 'y'
In this case stare is neuter gender and stary masculine. Stara - feminine. All singular.
Starzy=> masculine-personal plural. Stare=> non-masculine-personal plural.
As you can see stare can be both singular and plural. BUT adjectives always have to match nouns. So there should be no doubt when to use singular and when plural.